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Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2018	Feb. 26, 2019	Jun. 29, 2018
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY
Trading Symbol	WVE
Entity Registrant Name	WAVE LIFE SCIENCES LTD.
Entity Central Index Key	0001631574
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		34,192,106
Entity Public Float			$ 746,006,159

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 174,819	$ 142,503
Current portion of accounts receivable	10,000	1,000
Prepaid expenses and other current assets	17,454	6,985
Total current assets	202,273	150,488
Long-term assets:
Accounts receivable, net of current portion	50,000
Property and equipment, net	39,931	27,334
Restricted cash	3,625	3,610
Other assets	111	411
Total long-term assets	93,667	31,355
Total assets	295,940	181,843
Current liabilities:
Accounts payable	13,089	7,598
Accrued expenses and other current liabilities	14,736	8,898
Current portion of capital lease obligation		16
Current portion of deferred rent	115	60
Current portion of deferred revenue	100,945	1,275
Current portion of lease incentive obligation	1,156	344
Total current liabilities	130,041	18,191
Long-term liabilities:
Deferred rent, net of current portion	5,132	4,214
Deferred revenue, net of current portion	68,156	7,241
Lease incentive obligation, net of current portion	9,247	3,094
Other liabilities	2,142	1,619
Total long-term liabilities	84,677	16,168
Total liabilities	214,718	34,359
Series A preferred shares, no par value; 3,901,348 shares issued and outstanding at December 31, 2018 and 2017	7,874	7,874
Shareholders’ equity:
Ordinary shares, no par value; 29,472,197 and 27,829,079 shares issued and outstanding at December 31, 2018 and 2017, respectively	375,148	310,038
Additional paid-in capital	37,768	22,172
Accumulated other comprehensive income (loss)	153	116
Accumulated deficit	(339,721)	(192,716)
Total shareholders’ equity	73,348	139,610
Total liabilities, Series A preferred shares and shareholders’ equity	$ 295,940	$ 181,843

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Series A preferred stock, par value	$ 0	$ 0
Series A preferred stock, shares issued	3,901,348	3,901,348
Series A preferred stock, shares outstanding	3,901,348	3,901,348
Common stock, par value	$ 0	$ 0
Common stock, shares issued	29,472,197	27,829,079
Common stock, shares outstanding	29,472,197	27,829,079

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Revenue	$ 14,414	$ 3,893	$ 1,092
Operating expenses:
Research and development	134,428	79,309	40,818
General and administrative	39,509	26,975	15,994
Total operating expenses	173,937	106,284	56,812
Loss from operations	(159,523)	(102,391)	(55,720)
Other income (expense), net:
Dividend income	3,368	1,578	255
Interest income (expense), net	22	6	337
Other income (expense), net	9,549	(331)	(50)
Other income (expense), net	12,939	1,253	542
Loss before income taxes	(146,584)	(101,138)	(55,178)
Income tax benefit (provision)	(69)	(842)	(482)
Net loss	$ (146,653)	$ (101,980)	$ (55,660)
Net loss per share attributable to ordinary shareholders—basic and diluted	$ (5.06)	$ (3.85)	$ (2.44)
Weighted-average ordinary shares used in computing net loss per share attributable to ordinary shareholders—basic and diluted	28,970,404	26,513,382	22,800,628
Other comprehensive income (loss):
Net loss	$ (146,653)	$ (101,980)	$ (55,660)
Foreign currency translation	37	407	(332)
Comprehensive loss	$ (146,616)	$ (101,573)	$ (55,992)

Consolidated Statements of Series A Preferred Shares and Shareholders' Equity - USD ($) $ in Thousands	Total	Series A Preferred Shares [Member]	Ordinary Shares [Member]	Additional Paid-In-Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2015	$ 153,491		$ 185,344	$ 3,182	$ 41	$ (35,076)
Beginning balance, shares at Dec. 31, 2015			21,551,423
Temporary equity, Beginning balance at Dec. 31, 2015		$ 7,874
Temporary equity, Beginning balance, shares at Dec. 31, 2015		3,901,348
Issuance of ordinary shares, net of offering costs	30,000		$ 30,000
Issuance of ordinary shares, net of offering costs, shares			1,875,000
Share-based compensation	6,847			6,847
Option exercises	258		$ 258
Option exercises, shares			75,746
Other comprehensive income (loss)	(332)				(332)
Net loss	(55,660)					(55,660)
Ending balance at Dec. 31, 2016	134,604		$ 215,602	10,029	(291)	(90,736)
Ending balance, shares at Dec. 31, 2016			23,502,169
Temporary equity, Ending balance at Dec. 31, 2016		$ 7,874
Temporary equity, Ending balance, shares at Dec. 31, 2016		3,901,348
Issuance of ordinary shares, net of offering costs	93,509		$ 93,509
Issuance of ordinary shares, net of offering costs, shares			4,166,667
Share-based compensation	12,143			12,143
Vesting of RSUs, shares			22,750
Option exercises	927		$ 927
Option exercises, shares			137,493
Other comprehensive income (loss)	407				407
Net loss	(101,980)					(101,980)
Ending balance at Dec. 31, 2017	139,610		$ 310,038	22,172	116	(192,716)
Ending balance, shares at Dec. 31, 2017			27,829,079
Temporary equity, Ending balance at Dec. 31, 2017	$ 7,874	$ 7,874
Temporary equity, Ending balance, shares at Dec. 31, 2017	3,901,348	3,901,348
Issuance of ordinary shares, net of offering costs	$ 60,000		$ 60,000
Issuance of ordinary shares, net of offering costs, shares			1,096,892
Share-based compensation	15,596			15,596
Vesting of RSUs, shares			38,594
Option exercises	5,110		$ 5,110
Option exercises, shares			507,632
Impact of adoption of ASU 2016-16	(352)					(352)
Other comprehensive income (loss)	37				37
Net loss	(146,653)					(146,653)
Ending balance at Dec. 31, 2018	73,348		$ 375,148	$ 37,768	$ 153	$ (339,721)
Ending balance, shares at Dec. 31, 2018			29,472,197
Temporary equity, Ending balance at Dec. 31, 2018	$ 7,874	$ 7,874
Temporary equity, Ending balance, shares at Dec. 31, 2018	3,901,348	3,901,348

Consolidated Statements of Series A Preferred Shares and Shareholders' Equity (Parenthetical) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)
Ordinary Shares [Member]
Issuance costs	$ 491

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities
Net loss	$ (146,653)	$ (101,980)	$ (55,660)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Amortization of lease incentive obligation	(790)	(208)
Depreciation of property and equipment	5,581	2,155	784
Share-based compensation expense	15,596	12,143	6,847
Net (gain) loss on disposal of property and equipment	44	205
Deferred rent	973	3,594	565
Deferred income taxes		908	(698)
Tax benefit related to share-based compensation			(310)
Changes in operating assets and liabilities:
Accounts receivable	(59,000)	(1,000)
Prepaid expenses and other current assets	(10,469)	(5,502)	(1,337)
Other assets	(52)	(358)	(53)
Accounts payable	4,936	3,892	3,369
Accrued expenses and other current liabilities	5,864	4,550	2,296
Deferred revenue	160,585	(2,893)	11,408
Other non-current liabilities	523	823	864
Net cash used in operating activities	(22,862)	(83,671)	(31,925)
Cash flows from investing activities
Proceeds from the sale of property and equipment			4
Purchases of property and equipment, net of tenant improvement allowances	(9,938)	(18,887)	(5,567)
Net cash used in investing activities	(9,938)	(18,887)	(5,563)
Cash flows from financing activities
Costs associated with initial public offering			(1,075)
Proceeds from issuance of ordinary shares, net of offering costs	60,000	93,509	30,000
Payments on capital lease obligation	(16)	(62)	(62)
Proceeds from the exercise of share options	5,110	927	258
Net cash provided by financing activities	65,094	94,374	29,121
Effect of foreign exchange rates on cash	37	403	(14)
Net increase (decrease) in cash, cash equivalents and restricted cash	32,331	(7,781)	(8,381)
Cash, cash equivalents and restricted cash, beginning of period	146,113	153,894	162,275
Cash, cash equivalents and restricted cash, end of period	$ 178,444	$ 146,113	$ 153,894

The Company

12 Months Ended

Dec. 31, 2018

Organization Consolidation And Presentation Of Financial Statements [Abstract]

The Company

1. THE COMPANY

Organization

Wave Life Sciences Ltd. (together with its subsidiaries, “Wave” or the “Company”) is a clinical-stage genetic medicines company committed to delivering life-changing treatments for people battling devastating diseases. Using PRISM, Wave’s proprietary discovery and drug development platform, enables Wave to target genetically defined diseases with stereopure oligonucleotides across multiple therapeutic modalities.

The Company was incorporated in Singapore on July 23, 2012 and has its principal U.S. office in Cambridge, Massachusetts. The Company was incorporated with the purpose of combining two commonly held companies, Wave Life Sciences USA, Inc. (“Wave USA”), a Delaware corporation (formerly Ontorii, Inc.), and Wave Life Sciences Japan, Inc. (“Wave Japan”), a company organized under the laws of Japan (formerly Chiralgen., Ltd.), which occurred on September 13, 2012. On May 31, 2016, Wave Life Sciences Ireland Limited (“Wave Ireland”) was formed as a wholly-owned subsidiary of Wave Life Sciences Ltd. On April 3, 2017, Wave Life Sciences UK Limited (“Wave UK”) was formed as a wholly-owned subsidiary of Wave Life Sciences Ltd.

The Company’s primary activities since inception have been developing PRISM to design, develop and commercialize oligonucleotide therapeutics, advancing the Company’s neurology business, building the Company’s research and development activities in ophthalmology and hepatic, advancing programs into the clinic, furthering clinical development of such clinical-stage programs, building the Company’s intellectual property, and assuring adequate capital to support these activities.

Risks and Uncertainties

The Company is subject to risks common to companies in the biotechnology industry including, but not limited to, new technological innovations, protection of proprietary technology, developing internal manufacturing capabilities, dependence on key personnel, compliance with government regulations and the need to obtain additional financing. The Company’s therapeutic programs will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval, prior to commercialization of any product candidates. These efforts require significant amounts of additional capital, adequate personnel infrastructure and extensive compliance-reporting capabilities. There can be no assurance that the Company’s research and development efforts will be successful, that adequate protection for the Company’s intellectual property will be obtained, that any products developed will obtain necessary government regulatory approval or that any approved products will be commercially viable. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will generate significant revenue from product sales. The Company operates in an environment of rapid change in technology and substantial competition from pharmaceutical and biotechnology companies.

Basis of Presentation

The Company has prepared the accompanying consolidated financial statements in conformity with generally accepted accounting principles in the United States (“U.S. GAAP”) and in U.S. dollars.

Significant Accounting Policies

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Significant Accounting Policies

2. SIGNIFICANT ACCOUNTING POLICIES

Cash Equivalents

The Company considers all highly liquid securities with original final maturities of three months or less from the date of purchase to be cash equivalents. Cash equivalents are comprised of funds in money market accounts.

Principles of Consolidation

The Company’s consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation.

Use of Estimates

The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include the valuation of the Company’s ordinary shares prior to the initial public offering in November 2015, the assumptions used to determine the fair value of share-based awards, the Company’s revenue recognition policy, particularly, (a) assessing the number of performance obligations; (b) determining the transaction price; (c) allocating the transaction price to the performance obligations in the contract; and (d) determining the pattern over which performance obligations are satisfied, including estimates to complete performance obligations, the evaluation of progress to completion of external research and development costs which can result in prepaid or accrued expenses related to the Company’s contract research organizations (“CROs”) and contract manufacturing organizations (“CMOs”), and the valuation allowance required for the Company’s deferred tax assets and determining uncertain tax positions and the related liabilities. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Actual results could differ from the Company’s estimates.

Segment Data

The Company manages its operations as a single segment for the purposes of assessing performance and making operating decisions. The Company’s singular focus is on developing its proprietary discovery and drug development platform, PRISM, to develop and commercialize a broad pipeline of nucleic acid-based therapeutics, or oligonucleotides.

Foreign Currency Translation

The functional currency is the U.S. dollar for all of the Company’s entities aside from Wave Japan, which has the Japanese Yen as its functional currency. Assets and liabilities of Wave Japan are translated at period end exchange rates while revenues and expenses are translated at average exchange rates for the period. Net unrealized gains and losses from foreign currency translation are reflected as other comprehensive income (loss) within the consolidated statements of Series A preferred shares and shareholders’ equity and the consolidated statements of operations and comprehensive loss. Gains and losses on foreign currency transactions are included in the consolidated statements of operations and comprehensive loss within other income (expenses), net.

Fair Value of Financial Instruments

The Company is required to disclose information on all assets and liabilities reported at fair value that enables an assessment of the inputs used in determining the reported fair values. The fair value hierarchy is a hierarchy of inputs used in measuring fair value that maximizes the use of observable inputs and minimizes the use of unobservable inputs by requiring that the observable inputs be used when available. Observable inputs are inputs that market participants would use in pricing the financial instrument based on market data obtained from sources independent of the Company. Unobservable inputs are inputs that reflect the Company’s assumptions about the inputs that market participants would use in pricing the financial instrument and are developed based on the information available in the circumstances. The fair value hierarchy applies only to the valuation inputs used in determining the reported fair value of the investments and is not a measure of the investment credit quality. The hierarchy defines three levels of valuation inputs:

Level 1—Unadjusted quoted prices in active markets that are accessible at the measurement date of identical, unrestricted assets.

Level 2—Quoted prices for similar assets, or inputs that are observable, either directly or indirectly, for substantially the full term through corroboration with observable market data. Level 2 includes investments valued at quoted prices adjusted for legal or contractual restrictions specific to the security.

Level 3—Pricing inputs are unobservable for the asset, that is, inputs that reflect the reporting entity’s own assumptions about the assumptions market participants would use in pricing the asset. Level 3 includes private investments that are supported by little or no market activity.

Cash, cash equivalents and restricted cash are Level 1 assets which are comprised of funds held in checking and money market accounts. Cash, cash equivalents and restricted cash were recorded at fair value as of December 31, 2018 and 2017, totaling $178.4 million and $146.1 million, respectively. The carrying amounts of accounts payable and accrued expenses approximate their fair values due to their short-term maturities. Accounts receivable relate to the Company’s collaboration agreements.

Concentration of Credit Risk

Cash and cash equivalents are financial instruments that potentially subject the Company to concentration of credit risk. The Company uses several financial institutions to maintain its cash and cash equivalents, all of which are high quality, accredited financial institutions and, accordingly, such funds are subject to minimal credit risk. The Company has not experienced any losses in such accounts and management believes that the Company is not exposed to significant credit risk due to the financial position of the depository institutions in which those deposits are held. The Company has no financial instruments with off-balance sheet risk of loss.

Restricted Cash

Restricted cash consists primarily of cash placed in separate restricted bank accounts as required under the terms of the Company’s lease agreements for its Cambridge, Massachusetts and Lexington, Massachusetts facilities (refer to Note 8). As of December 31, 2018 and 2017, the Company had $3.6 million of restricted cash, of which $2.6 million related to the Lexington facility and $1.0 million related to the Cambridge facility.

Property and Equipment

Property and equipment, which consists primarily of equipment, furniture, software and leasehold improvements, are stated at cost less accumulated depreciation. Depreciation is calculated on a straight-line basis over the following estimated useful lives of the assets:

Equipment, Furniture and Software	3-7 years
Leasehold Improvements	Shorter of asset life or lease term

Depreciation begins at the time the asset is placed in service. Maintenance and repairs are charged to operations as incurred. Upon retirement or sale, the cost of the disposed asset and the related accumulated depreciation are removed from the accounts and any resulting gain or loss is reflected in the consolidated statements of operations and comprehensive loss.

Impairment of Long-Lived Assets

Long-lived assets consist of property and equipment. Long-lived assets are reviewed for impairment whenever events or other changes in circumstances indicate that the carrying amount may not be recoverable. Certain factors may exist or events may occur that indicate that impairment exists including, but not limited to, the following: significant underperformance relative to historical or projected future operating results; significant changes in the manner of use of the underlying assets; and significant adverse industry or market economic trends.

When performing the impairment assessment for long-lived assets, the Company compares the carrying value of such assets to the estimated undiscounted future net cash flows expected from the use of the assets and their eventual disposition. In the event that the carrying value of the assets is determined to be unrecoverable, the Company would estimate the fair value of the assets and record an impairment charge for the excess of the carrying value over the fair value.

Revenue Recognition

Effective January 1, 2018, the Company adopted Accounting Standards Codification (“ASC”) Topic 606, Revenue from Contracts with Customers (“ASC 606”), using the full retrospective transition method. Under this method, the Company revised its consolidated financial statements for the years ended December 31, 2017 and 2016, and applicable interim periods within those years, as if ASC 606 had been effective for those periods. This standard applies to all contracts with customers, except for contracts that are within the scope of other standards, such as leases, insurance, and financial instruments. Under ASC 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration that the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of ASC 606, the entity performs the following five-step analysis: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the entity satisfies a performance obligation. The Company only applies the five-step analysis to contracts when it is probable that the entity will collect the consideration to which it is entitled in exchange for the goods or services it transfers to the customer. At contract inception, once the contract is determined to be within the scope of ASC 606, the Company assesses the goods or services promised within each contract, determines those that are performance obligations, and assesses whether each promised good or service is distinct. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied.

The Company has entered into collaboration agreements for research, development, and commercial services, under which the Company licenses certain rights to its product candidates to third parties. The terms of these arrangements typically include payment to the Company of one or more of the following: non-refundable, upfront license fees; reimbursement of certain costs; customer option exercise fees; development, regulatory and commercial milestone payments; and royalties on net sales of licensed products. Any variable consideration is constrained and, therefore, the cumulative revenue associated with this consideration is not recognized until it is deemed not to be at significant risk of reversal.

In determining the appropriate amount of revenue to be recognized as the Company fulfills its obligations under each of its agreements for which the collaboration partner is also a customer, the Company performs the following steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations, including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation. As part of the accounting for these arrangements, the Company must use significant judgment to determine: (a) the number of performance obligations based on the determination under step (ii) above; (b) the transaction price under step (iii) above; and (c) the timing of satisfaction of performance obligations as a measure of progress in step (v) above. The Company uses significant judgment to determine whether milestones or other variable consideration, except for royalties, should be included in the transaction price as described further below. The transaction price is allocated to the optional goods and services the Company expects to provide. The Company uses estimates to determine the timing of satisfaction of performance obligations.

Amounts received prior to being recognized as revenue are recorded as deferred revenue. Amounts expected to be recognized as revenue within the 12 months following the balance sheet date are classified as current portion of deferred revenue in the accompanying consolidated balance sheets. Amounts not expected to be recognized as revenue within the 12 months following the balance sheet date are classified as deferred revenue, net of current portion.

Licenses of intellectual property: In assessing whether a promise or performance obligation is distinct from the other promises, the Company considers factors such as the research, development, manufacturing and commercialization capabilities of the customer and the availability of the associated expertise in the general marketplace. In addition, the Company considers whether the customer can benefit from a promise for its intended purpose without the receipt of the remaining promise, whether the value of the promise is dependent on the unsatisfied promise, whether there are other vendors that could provide the remaining promise, and whether it is separately identifiable from the remaining promise. For licenses that are combined with other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition.

Research and development services: If an arrangement is determined to contain a promise or obligation for the Company to perform research and development services, the Company must determine whether these services are distinct from other promises in the arrangement. In assessing whether the services are distinct from the other promises, the Company considers the capabilities of the customer to perform these same services. In addition, the Company considers whether the customer can benefit from a promise for its intended purpose without the receipt of the remaining promise, whether the value of the promise is dependent on the unsatisfied promise, whether there are other vendors that could provide the remaining promise, and whether it is separately identifiable from the remaining promise. For research and development services that are combined with other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition.

Customer options: If an arrangement is determined to contain customer options that allow the customer to acquire additional goods or services, the goods and services underlying the customer options are not considered to be performance obligations at the outset of the arrangement, as they are contingent upon option exercise. The Company evaluates the customer options for material rights, that is, the option to acquire additional goods or services for free or at a discount. If the customer options are determined to represent a material right, the material right is recognized as a separate performance obligation at the outset of the arrangement. The Company allocates the transaction price to material rights based on the standalone selling price. As a practical alternative to estimating the standalone selling price when the goods or services are both (i) similar to the original goods and services in the contract and (ii) provided in accordance with the terms of the original contract, the Company allocates the total amount of consideration expected to be received from the customer to the total goods or services expected to be provided to the customer. Amounts allocated to any material right are not recognized as revenue until the option is exercised and the performance obligation is satisfied.

Milestone payments: At the inception of each arrangement that includes milestone payments, the Company evaluates whether a significant reversal of cumulative revenue provided in conjunction with achieving the milestones is probable, and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant reversal of cumulative revenue would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the control of the Company or the licensee, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. For other milestones, the Company evaluates factors such as the scientific, clinical, regulatory, commercial, and other risks that must be overcome to achieve the particular milestone in making this assessment. There is considerable judgment involved in determining whether it is probable that a significant reversal of cumulative revenue would not occur. At the end of each subsequent reporting period, the Company reevaluates the probability of achievement of all milestones subject to constraint and, if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect revenues and earnings in the period of adjustment.

Royalties: For arrangements that include sales-based royalties, including milestone payments based on a level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). To date, the Company has not recognized any royalty revenue resulting from any of its licensing arrangements.

Contract costs: The Company recognizes as an asset the incremental costs of obtaining a contract with a customer if the costs are expected to be recovered. As a practical expedient, the Company recognizes the incremental costs of obtaining a contract as an expense when incurred if the amortization period of the asset that it otherwise would have recognized is one year or less. To date, the Company has not incurred any incremental costs of obtaining a contract with a customer.

For additional discussion of accounting for collaboration revenues, see Note 5.

Product Revenue

The Company has had no product revenue to date.

Research and Development Expenses

Research and development expenses are expensed as incurred. External development costs are recognized based on an evaluation of the progress to completion of specific tasks. Payments for these activities are based on the terms of the individual agreements, which may differ from the pattern of costs incurred, and are reflected in the accompanying consolidated balance sheets as prepaid or accrued expenses.

Refundable Tax Credits

The Company is eligible for refundable tax credits with tax authorities for certain qualified operating expenses. The Company recognizes refundable tax credits when there is reasonable assurance that the Company will comply with the requirements of the refundable tax credit and that the refundable tax credit will be received.

Refundable tax credits are recorded as income and classified in other income (expense), net in the consolidated statements of operations and comprehensive loss.

Net Loss per Share

Basic net loss per share is computed using the weighted-average number of ordinary shares outstanding during the period. Diluted net loss per share is computed using the sum of the weighted-average number of ordinary shares outstanding during the period and, if dilutive, the weighted-average number of potential ordinary shares, including the assumed exercise of share options.

The Company applies the two-class method to calculate its basic and diluted net loss per share attributable to ordinary shareholders, as its Series A preferred shares are participating securities. The two-class method is an earnings allocation formula that treats a participating security as having rights to earnings that otherwise would have been available to ordinary shareholders. However, for the periods presented, the two-class method does not impact the net loss per ordinary share as the Company was in a net loss position for each of the periods presented and holders of Series A preferred shares do not participate in losses.

The Company’s Series A preferred shares contractually entitle the holders of such shares to participate in dividends but do not contractually require the holders of such shares to participate in losses of the Company. Accordingly, for periods in which the Company reports a net loss attributable to ordinary shareholders, diluted net loss per share attributable to ordinary shareholders is the same as basic net loss per share attributable to ordinary shareholders, since dilutive ordinary shares are not assumed to have been issued if their effect is anti-dilutive.

License Agreements and Patent Costs

Costs associated with licenses of technology and patent costs are expensed as incurred and are generally included in research and development expense in the consolidated statements of operations and comprehensive loss.

Share-Based Compensation

The Company measures and recognizes share-based compensation expense, for both employee and director option awards, based on the grant date fair value of the awards. The Company calculates the fair value of restricted share unit awards based on the grant date fair value of the underlying ordinary shares. The Company recognizes share-based compensation expense on a straight-line basis over the requisite service period of the awards, which is generally the vesting period.

The Company determines the fair value of share-based awards granted to non-employees as either the fair value of the consideration received or the fair value of the equity instruments issued, whichever is more reliably measurable. All equity instruments issued to non-employees as consideration for goods or services received by the Company are accounted for based on the fair value of the equity instruments issued. These awards are recorded in expense and additional paid-in capital in shareholders’ equity over the applicable service periods based on the fair value of the options at the end of each period. The Company accounts for the expense from share-based awards to non-employees by re-measuring the awards at fair value over the vesting period.

The Company classifies share-based compensation expense in its consolidated statements of operations and comprehensive loss in the same manner in which the award recipient’s compensation costs are classified or in which the award recipient’s service payments are classified.

The fair value of each share option grant was determined using the methods and assumptions discussed below. These inputs are generally subjective and require significant judgment and estimation by management.

•

Fair Value of Ordinary Shares The fair value of the ordinary shares underlying the Company’s share-based awards is based on the closing price of the Company’s ordinary shares as reported by the Nasdaq Global Market on the date of grant.

•

Expected Term The expected term of share options represents the weighted-average period that the share options are expected to remain outstanding. The Company estimated the expected term using the simplified method, which is an average of the contractual term of the option and the vesting period.

•

Expected Volatility Since there is limited historical data for the Company’s ordinary shares and limited company-specific historical volatility, it has determined the share price volatility for options granted based on an analysis of the volatility used by a peer group of publicly traded companies. In evaluating similarity, the Company considers factors such as industry, stage of life cycle and size.

•

Risk-free Interest Rate The risk-free interest rate is based on the U.S. Treasury yield in effect at the time of the grant for zero-coupon U.S. Treasury notes with remaining terms similar to the expected term of the options.

•

Dividend Rate The expected dividend was assumed to be zero as the Company has never paid dividends and has no current plans to do so.

Income Taxes

The Company accounts for income taxes using an asset and liability approach, which requires recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the consolidated financial statements, but have not been reflected in taxable income. A valuation allowance is established to reduce deferred tax assets to their estimated realizable value. Therefore, the Company provides a valuation allowance to the extent that it is more likely than not that all or a portion of the deferred tax assets will not be realized in the future.

The Company accounts for uncertainty in income taxes recognized in the financial statements by applying a two-step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the tax authorities. If the tax position is deemed more-likely-than-not to be sustained, the tax position is then assessed to determine the amount of benefit to recognize in the financial statements. The amount of the benefit that may be recognized is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties.

The Company recognizes interest and penalties related to uncertain tax positions in the income tax provision on the consolidated statements of operations and comprehensive loss.

The Company has certain service arrangements in place between its U.S., Japan, UK and Singapore entities, which include transfer pricing assumptions. The determination of the appropriate level of transfer pricing requires judgment based on transfer pricing analyses of comparable companies. The Company monitors the nature of its service arrangements for changes in its operations as well as economic conditions. The Company also periodically reviews the transfer pricing analyses for changes in the composition in the pool of comparable companies as well as the related ongoing results of the comparable companies.

Recently Issued Accounting Pronouncements

In February 2016, the Financial Accounting Standards Board (the “FASB”) issued Accounting Standards Update (“ASU”) No. 2016-02, Leases (“ASU 2016-02”), which was further clarified in July 2018 when the FASB issued Accounting Standards Update No. 2018-10, Codification Improvements to Topic 842, Leases (“ASU 2018-10”) and Accounting Standards Update No. 2018-11, Leases (Topic 842)—Targeted Improvements (“ASU 2018-11”). ASU 2016-02, ASU 2018-10 and ASU 2018-11 require a lessee to recognize assets and liabilities on the balance sheet for operating leases and changes many key definitions, including the definition of a lease. The update includes a short-term lease exception for leases with a term of 12 months or less, in which a lessee can make an accounting policy election not to recognize lease assets and lease liabilities. Lessees will continue to differentiate between finance leases (previously referred to as capital leases) and operating leases, using classification criteria that are substantially similar to the previous guidance. For lessees, the recognition, measurement, and presentation of expenses and cash flows arising from a lease have not significantly changed from previous U.S. GAAP. Lessees and lessors are required to recognize and measure leases at the beginning of the earliest period presented using a modified retrospective approach. The modified retrospective approach includes a number of optional practical expedients that entities may elect to apply, as well as transition guidance specific to nonstandard leasing transactions. ASU 2016-02, ASU 2018-10 and ASU 2018-11 are effective for fiscal years beginning after December 15, 2018, and interim periods within those fiscal years. The adoption of the new standard will result in the recognition of a material right-of-use asset and lease liability on the Company’s consolidated balance sheet. The Company does not expect the adoption of the new standard to have a material impact on its consolidated statement of operations or its liquidity.

In February 2018, the FASB issued Accounting Standards Update No. 2018-02, Income Statement—Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income (“ASU 2018-02”), which allows companies to make a one-time reclassification of the stranded tax effects (as defined by ASU 2018-02) from accumulated other comprehensive income to retained earnings as a result of the tax legislation enacted in December 2017, commonly known as the “Tax Cuts and Jobs Act” (the “Tax Act”), and requires certain disclosures about the stranded tax effects. The new guidance is effective for fiscal years beginning after December 15, 2018, including interim periods within those fiscal years. The Company is currently evaluating the potential impact that the adoption of ASU 2018-02 may have on its consolidated financial statements.

In March 2018, the FASB issued Accounting Standards Update No. 2018-05, Income Taxes (Topic 740): Amendments to SEC Paragraphs Pursuant to SEC Staff Accounting Bulletin No. 118 (“ASU 2018-05”). The standard amends Accounting Standards Codification 740, Income Taxes (“ASC 740”), to provide guidance on accounting for the tax effects of the Tax Act pursuant to Staff Accounting Bulletin No. 118. As of December 31, 2018, the Company has finalized its accounting for the impact of 2018-05. In 2018, the Company recognized a $0.2 million benefit related to the remeasurement of its deferred tax assets and liabilities as a result of filing its 2017 U.S. tax return.

In November 2018, the FASB issued Accounting Standards Update No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 (“ASU 2018-18”). The standard amends Accounting Standards Codification 808, Collaborative Agreements and Accounting Standards Codification 606, Revenue from Contracts with Customers, to clarify the interaction between collaborative arrangement participants that should be accounted for as revenue under ASC 606. In transactions when the collaborative arrangement participant is a customer in the context of a unit of account, revenue should be accounted for using the guidance in Topic 606. The amendments in Update No. 2018-18 are effective for fiscal years beginning after December 15, 2019, and interim periods within those fiscal years. The Company is currently evaluating the new guidance included in ASU 2018-18, but does not expect it to have a material impact on its consolidated financial statements.

Recently Adopted Accounting Pronouncements

In May 2014, the FASB issued Accounting Standards Update No. 2014-09, which amends the guidance for accounting for revenue from contracts with customers. This ASU supersedes the revenue recognition requirements in ASC Topic 605, Revenue Recognition, (“ASC 605”), and creates a new topic, ASC 606, Revenue from Contracts with Customers. In 2015 and 2016, the FASB issued additional ASUs related to ASC 606 that delayed the effective date of the guidance and clarified various aspects of the new revenue guidance, including principal versus agent considerations, identifying performance obligations, and licensing, and they include other improvements and practical expedients. The Company adopted this new standard on January 1, 2018 using the full retrospective transition method.

Impact of Adoption

As a result of adopting ASC 606 on January 1, 2018, the Company has revised its comparative financial statements for the prior year as if ASC 606 had been effective for that period. As a result, the following financial statement line items were affected as of December 31, 2017 and for the years ended December 31, 2017 and 2016.

Consolidated Balance Sheets
	As of December 31, 2017
	As revised under ASC 606			As originally reported under ASC 605			Effect of change
	(in thousands)
Current portion of deferred revenue	$	1,275		$	2,705		$	(1,430	)
Deferred revenue, net of current portion		7,241			5,607			1,634
Accumulated deficit		(192,716	)		(192,512	)		(204	)

Consolidated Statements of Operations and Comprehensive Loss

	For the Year Ended December 31, 2017
	As revised under ASC 606			As originally reported under ASC 605			Effect of change
	(in thousands, except per share data)
Revenue	$	3,893		$	3,704		$	189
Loss from operations		(102,391	)		(102,580	)		189
Income tax benefit (provision)		(842	)		(708	)		(134	)
Net loss		(101,980	)		(102,035	)		55

	For the Year Ended December 31, 2016
	As revised under ASC 606			As originally reported under ASC 605			Effect of change
	(in thousands, except per share data)
Revenue	$	1,092		$	1,485		$	(393	)
Loss from operations		(55,720	)		(55,327	)		(393	)
Income tax benefit (provision)		(482	)		(616	)		134
Net loss		(55,660	)		(55,401	)		(259	)
Net loss per share attributable to ordinary shareholders—basic and diluted	$	(2.44	)	$	(2.43	)	$	(0.01	)

Consolidated Statement of Cash Flows
	For the Year Ended December 31, 2017
	As revised under ASC 606			As originally reported under ASC 605			Effect of change
	(in thousands)
Net loss	$	(101,980	)	$	(102,035	)	$	55
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Deferred income taxes		908			774			134
Changes in operating assets and liabilities:
Deferred revenue		(2,893	)		(2,704	)		(189	)

	For the Year Ended December 31, 2016
	As revised under ASC 606			As originally reported under ASC 605			Effect of change
	(in thousands)
Net loss	$	(55,660	)	$	(55,401	)	$	(259	)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Deferred income taxes		(698	)		(564	)		(134	)
Changes in operating assets and liabilities:
Deferred revenue		11,408			11,015			393

The most significant changes relate to the Company’s revenue recognition pattern for the Pfizer Collaboration Agreement (as defined in Note 5) and the accounting for milestone payments.

Under ASC 605, the Company was recognizing the revenue allocated to each unit of accounting on a straight-line basis over the period the Company is expected to complete its obligations. Under ASC 606, the Company is recognizing the revenue allocated to each performance obligation over time, measuring progress using an input method over the period the Company is expected to complete each performance obligation.

Under ASC 605, the Company recognized revenue related to milestone payments as the milestone was achieved, using the milestone method. Under ASC 606, the Company performs an assessment of the probability of milestone achievement at each reporting date and determines whether the cumulative revenue related to the milestone is at risk of significant reversal.

In October 2016, the FASB issued Accounting Standards Update No. 2016-16, Income Taxes (Topic 740): Intra-Entity Transfers of Assets Other Than Inventory (“ASU 2016-16”). Under the new guidance, companies are required to recognize the income tax consequences of an intra-entity transfer of an asset, other than inventory, when the transfer occurs, even though the pre-tax effects of that transaction are eliminated in consolidation. The amendments are effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2017. The Company adopted ASU 2016-16 effective January 1, 2018, which resulted in a $0.4 million cumulative-effect adjustment to retained earnings.

In November 2016, the FASB issued Accounting Standards Update No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash (“ASU 2016-18”). ASU 2016-18 requires that an entity explain the changes in the total of cash, cash equivalents, restricted cash and restricted cash equivalents on the statement of cash flows. Therefore, amounts generally described as restricted cash and restricted cash equivalents should be included with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. This ASU is effective for annual and interim periods beginning after December 15, 2017. The Company adopted ASU 2016-18 effective January 1, 2018 on a retrospective basis, which resulted in a change in presentation of restricted cash within the Company’s consolidated statements of cash flows.

Property and Equipment, Net

12 Months Ended

Dec. 31, 2018

Property Plant And Equipment [Abstract]

Property and Equipment, Net

3. PROPERTY AND EQUIPMENT, NET

Property and equipment, net, consists of the following:

	December 31,
	2018			2017
	(in thousands)
Furniture and equipment	$	20,300		$	13,626
Software		435			108
Leasehold improvements		27,342			16,029
Fixed assets in progress		1,354			1,988
Total		49,431			31,751
Less accumulated depreciation		(9,500	)		(4,417	)
Property and equipment, net	$	39,931		$	27,334

Depreciation expense was $5.6 million, $2.2 million and $0.8 million for the years ended December 31, 2018, 2017 and 2016, respectively.

Accrued Expenses and Other Current Liabilities

12 Months Ended

Dec. 31, 2018

Payables And Accruals [Abstract]

Accrued Expenses and Other Current Liabilities

4. ACCRUED EXPENSES AND OTHER CURRENT LIABILITIES

Accrued expenses and other current liabilities consist of the following:

	December 31,
	2018		2017
	(in thousands)
Accrued compensation	$	7,507	$	5,461
Accrued expenses related to CROs and CMOs		5,502		3,281
Accrued expenses and other current liabilities		1,727		156
Total accrued expenses and other current liabilities	$	14,736	$	8,898

Collaboration Agreements

12 Months Ended

Dec. 31, 2018

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Collaboration Agreements

5. COLLABORATION AGREEMENTS

Pfizer Collaboration and Equity Agreements

In May 2016, the Company entered into a Research, License and Option Agreement (as amended in November 2017, the “Pfizer Collaboration Agreement”) with Pfizer Inc. (“Pfizer”). Pursuant to the terms of the Pfizer Collaboration Agreement, the Company and Pfizer agreed to collaborate on the discovery, development and commercialization of stereopure oligonucleotide therapeutics for up to five programs (the “Pfizer Programs”), each directed at a genetically-defined hepatic target selected by Pfizer (the “Pfizer Collaboration”). The Company received $10.0 million as an upfront license fee under the Pfizer Collaboration Agreement. Subject to option exercises by Pfizer, the Company may earn potential research, development and commercial milestone payments, plus royalties, tiered up to low double-digits, on sales of any products that may result from the Pfizer Collaboration. None of the payments under the Pfizer Collaboration Agreement are refundable.

Simultaneously with the entry into the Pfizer Collaboration Agreement, the Company entered into a Share Purchase Agreement (the “Pfizer Equity Agreement,” and together with the Pfizer Collaboration Agreement, the “Pfizer Agreements”) with C.P. Pharmaceuticals International C.V., an affiliate of Pfizer (the “Pfizer Affiliate”). Pursuant to the terms of the Pfizer Equity Agreement, the Pfizer Affiliate purchased 1,875,000 of the Company’s ordinary shares (the “Shares”) at a purchase price of $16.00 per share, for an aggregate purchase price of $30.0 million. The Company did not incur any material costs in connection with the issuance of the Shares.

Under the Pfizer Collaboration Agreement, the parties agreed to collaborate during a four-year research term. During the research term, the Company is responsible to use its commercially reasonable efforts to advance up to five programs through to the selection of clinical candidates. At that stage, Pfizer may elect to license any of these Pfizer Programs exclusively and obtain exclusive rights to undertake the clinical development of the resulting clinical candidates into products and the potential commercialization of any such products thereafter. In addition, the Company received a non-exclusive, royalty-bearing sublicensable license to use Pfizer’s hepatic targeting technology in any of the Company’s own hepatic programs that are outside the scope of the Pfizer Collaboration (the “Wave Programs”). If the Company uses this technology on the Wave Programs, Pfizer is eligible to receive potential development and commercial milestone payments from the Company. Pfizer is also eligible to receive tiered royalties on sales of any products that include Pfizer’s hepatic targeting technology.

The stated term of the Pfizer Collaboration Agreement commenced on May 5, 2016 and terminates on the date of the last to expire payment obligation with respect to each Pfizer Program and, with respect to each Wave Program, expires on a program-by-program basis accordingly. Pfizer may terminate its rights related to a Pfizer Program under the Pfizer Collaboration Agreement at its own convenience upon 90 days’ notice to the Company. The Company may also terminate its rights related to a Wave Program at its own convenience upon 90 days’ notice to Pfizer. The Pfizer Collaboration Agreement may also be terminated by either party in the event of an uncured material breach of the Pfizer Collaboration Agreement by the other party.

Pfizer nominated two hepatic targets upon entry into the Pfizer Collaboration in May 2016. The Pfizer Collaboration Agreement provides Pfizer with options to nominate up to three additional programs by making nomination milestone payments. Pfizer nominated the third, fourth and fifth hepatic targets in August 2016, March 2018 and April 2018, respectively.

The Pfizer Collaboration is managed by a joint steering committee in which both parties are represented equally, which will oversee the scientific progression of each Pfizer Program up to the clinical candidate stage. During the four-year research term and for a period of two years thereafter, the Company has agreed to work exclusively with Pfizer with respect to using any of the Company’s stereopure oligonucleotide technology that is specific for the applicable hepatic target which is the basis of any Pfizer Program. Within a specified period after receiving a data package for a candidate under each nominated program, Pfizer may exercise an option to obtain a license to develop, manufacture and commercialize the program candidate by paying an exercise price per program.

The Company assessed this arrangement in accordance with ASC 606 and concluded that the contract counterparty, Pfizer, is a customer. The Company identified the following promises under the arrangement: (1) the non-exclusive, royalty-free research and development license; (2) the research and development services for Programs 1 and 2; (3) the program nomination options for Programs 3, 4 and 5; (4) the research and development services associated with Programs 3, 4 and 5; (5) the options to obtain a license to develop, manufacture and commercialize Programs 1 and 2; and (6) the options to obtain a license to develop, manufacture and commercialize Programs 3, 4 and 5. The research and development services for each of Programs 1 and 2 were determined to not be distinct from the research and development license and should be combined into a single performance obligation for each program. The promises under the Pfizer Collaboration Agreement relate primarily to the research and development required by the Company for each of the programs nominated by Pfizer.

Additionally, the Company determined that the program nomination options for Programs 3, 4 and 5 were priced at a discount and, as such, provide material rights to Pfizer, representing three separate performance obligations. The research and development services associated with Programs 3, 4 and 5 and the options to obtain a license to develop, manufacture and commercialize Programs 3, 4 and 5 are subject to Pfizer’s exercise of the program nomination options for such programs and therefore do not represent performance obligations at the outset of the arrangement. The options to obtain a license to develop, manufacture and commercialize Programs 1 and 2 do not represent material rights; as such, they are not representative of performance obligations at the outset of the arrangement. Based on these assessments, the Company identified five performance obligations in the Pfizer Collaboration Agreement: (1) research and development services and license for Program 1; (2) research and development services and license for Program 2; (3) material right provided for the option to nominate Program 3; (4) material right provided for the option to nominate Program 4; and (5) material right provided for the option to nominate Program 5.

At the outset of the arrangement, the transaction price included only the $10.0 million up-front consideration received. The Company determined that the Pfizer Collaboration Agreement did not contain a significant financing component. The program nomination option exercise fees for research and development services associated with Programs 3, 4 and 5 that may be received are excluded from the transaction price until each customer option is exercised. The potential milestone payments were excluded from the transaction price, as all milestone amounts were fully constrained at the inception of the Pfizer Collaboration Agreement. The exercise fees for the options to obtain a license to develop, manufacture and commercialize Programs 3, 4 and 5 that may be received are excluded from the transaction price until each customer option is exercised. The Company will reevaluate the transaction price at the end of each reporting period and as uncertain events are resolved or other changes in circumstances occur, and, if necessary, will adjust its estimate of the transaction price.

During the year ended December 31, 2017, it became probable that a significant reversal of cumulative revenue would not occur for a developmental milestone under the Pfizer Collaboration Agreement. At such time, the associated consideration was added to the estimated transaction price and allocated to the existing performance obligations, and the Company recognized a cumulative catch-up to revenue for this developmental milestone, representing the amount that would have been recognized had the milestone payment been included in the transaction price from the outset of the arrangement. The remainder will be recognized in the same manner as the remaining, unrecognized transaction price over the remaining period until each performance obligation is satisfied.

Revenue associated with the performance obligations relating to Programs 1 and 2 is being recognized as revenue as the research and development services are provided using an input method, according to the full-time employee (“FTE”) hours incurred on each program and the FTE hours expected to be incurred in the future to satisfy the performance obligation. The transfer of control occurs over time and, in management’s judgment, this input method is the best measure of progress towards satisfying the performance obligation. The amount allocated to the three material rights will be recognized as the underlying research and development services are provided commencing from the date that Pfizer exercises each respective option, or immediately as each option expires unexercised. The amounts received that have not yet been recognized as revenue are recorded in deferred revenue on the Company’s consolidated balance sheet.

Pfizer nominated the third, fourth and fifth hepatic targets in August 2016, March 2018 and April 2018, respectively. Upon each exercise, the Company allocated the transaction price amount allocated to the material right at inception of the arrangement plus the program nomination option exercise fee paid by Pfizer at the time of exercising the option to a new performance obligation, which will be recognized as revenue as the research and development services are provided using the same method as the performance obligations relating to Programs 1 and 2.

Through December 31, 2018, the Company had recognized revenue of $9.9 million as collaboration revenue in the Company’s consolidated statements of operations and comprehensive loss under the Pfizer Collaboration Agreement. During the years ended December 31, 2018, 2017 and 2016, the Company recognized revenue of $4.9 million, $3.9 million and $1.1 million, respectively, under the Pfizer Collaboration Agreement. The aggregate amount of the transaction price allocated to the Company’s partially unsatisfied performance obligations and recorded in deferred revenue at December 31, 2018 is $8.7 million, of which $6.8 million is included in current liabilities. The Company expects to recognize the remaining deferred revenue according to FTE hours incurred, over the remaining research term, which is 16 months as of December 31, 2018.

Takeda Collaboration and Equity Agreements

In February 2018, Wave USA and Wave UK entered into a global strategic collaboration (the “Takeda Collaboration”) with Takeda Pharmaceutical Company Limited (“Takeda”), pursuant to which Wave USA, Wave UK and Takeda agreed to collaborate on the research, development and commercialization of oligonucleotide therapeutics for disorders of the Central Nervous System (“CNS”). The Takeda Collaboration provides Wave with at least $230.0 million in committed cash and Takeda with the option to co-develop and co-commercialize Wave’s CNS development programs in (1) Huntington’s disease (“HD”); (2) amyotrophic lateral sclerosis (“ALS”) and frontotemporal dementia (“FTD”); and (3) Wave’s discovery-stage program targeting ATXN3 for the treatment of spinocerebellar ataxia 3 (“SCA3”) (collectively, “Category 1 Programs”). In addition, Takeda will have the right to exclusively license multiple preclinical programs for CNS disorders, including Alzheimer’s disease and Parkinson’s disease (collectively, “Category 2 Programs”). In April 2018, the Takeda Collaboration became effective and Takeda paid Wave $110.0 million as an upfront payment. Takeda also agreed to fund Wave’s research and preclinical activities in the amount of $60.0 million during the four-year research term and to reimburse Wave for any collaboration-budgeted research and preclinical expenses incurred by Wave that exceed that amount.

Simultaneously with Wave USA and Wave UK’s entry into the collaboration and license agreement with Takeda (the “Takeda Collaboration Agreement”), the Company entered into a share purchase agreement with Takeda (the “Takeda Equity Agreement,” and together with the Takeda Collaboration Agreement, the “Takeda Agreements”) pursuant to which it agreed to sell to Takeda 1,096,892 of its ordinary shares at a purchase price of $54.70 per share. In April 2018, the Company closed the Takeda Equity Agreement and received aggregate cash proceeds of $60.0 million. The Company did not incur any material costs in connection with the issuance of shares.

With respect to Category 1 Programs, Wave will be responsible for researching and developing products and companion diagnostics for Category 1 Programs through completion of the first proof of mechanism study for such products. Takeda will have an exclusive option for each target and all associated products and companion diagnostics for such target, which it may exercise at any time through completion of the proof of mechanism study. If Takeda exercises this option, Wave will receive an opt-in payment and will lead manufacturing and joint clinical co-development activities and Takeda will lead joint co-commercial activities in the United States and all commercial activities outside of the United States. Global costs and potential profits will be shared 50:50 and Wave will be eligible to receive development and commercial milestone payments. In addition to its 50% profit share, Wave is eligible to receive option exercise fees and development and commercial milestone payments for each of the Category 1 Programs.

With respect to Category 2 Programs, Wave has granted Takeda the right to exclusively license multiple preclinical programs during a four-year research term (subject to limited extension for programs that were initiated prior to the expiration of the research term, in accordance with the Takeda Collaboration Agreement) (“Category 2 Research Term”). During that term, the parties may collaborate on preclinical programs for up to six targets at any one time. Wave will be responsible for researching and preclinically developing products and companion diagnostics directed to the agreed upon targets through completion of IND-enabling studies in the first major market country. Thereafter, Takeda will have an exclusive worldwide license to develop and commercialize products and companion diagnostics directed to such targets, subject to Wave’s retained rights to lead manufacturing activities for products directed to such targets. Takeda will fund Wave’s research and preclinical activities in the amount of $60.0 million during the research term and will reimburse Wave for any collaboration-budgeted research and preclinical expenses incurred by Wave that exceed that amount. Wave is also eligible to receive tiered high single-digit to mid-teen royalties on Takeda’s global commercial sales of products from each Category 2 Program.

Under the Takeda Collaboration Agreement, each party grants to the other party specific intellectual property licenses to enable the other party to perform its obligations and exercise its rights under the Takeda Collaboration Agreement, including license grants to enable each party to conduct research, development and commercialization activities pursuant to the terms of the Takeda Collaboration Agreement.

The term of the Takeda Collaboration Agreement commenced on April 2, 2018 and, unless terminated earlier, will continue until the date on which: (i) with respect to each Category 1 Program target for which Takeda does not exercise its option, the expiration or termination of the development program with respect to such target; (ii) with respect to each Category 1 Program target for which Takeda exercises its option, the date on which neither party is researching, developing or manufacturing any products or companion diagnostics directed to such target; or (iii) with respect to each Category 2 Program target, the date on which royalties are no longer payable with respect to products directed to such target.

Takeda may terminate the Takeda Collaboration Agreement for convenience on 180 days’ notice, in its entirety or on a target-by-target basis. Subject to certain exceptions, each party has the right to terminate the Takeda Collaboration Agreement on a target-by-target basis if the other party, or a third party related to such party, challenges the patentability, enforceability or validity of any patents within the licensed technology that cover any product or companion diagnostic that is subject to the Takeda Collaboration Agreement. In the event of any material breach of the Takeda Collaboration Agreement by a party, subject to cure rights, the other party may terminate the Takeda Collaboration Agreement in its entirety if the breach relates to all targets or on a target-by-target basis if the breach relates to a specific target. In the event that Takeda and its affiliates cease development, manufacturing and commercialization activities with respect to compounds or products subject to the Takeda Collaboration Agreement and directed to a particular target, Wave may terminate the Takeda Collaboration Agreement with respect to such target. Either party may terminate the Takeda Collaboration Agreement for the other party’s insolvency. In certain termination circumstances, Wave would receive a license from Takeda to continue researching, developing and manufacturing certain products, and companion diagnostics.

The Company assessed this arrangement in accordance with ASC 606 and concluded that the contract counterparty, Takeda, is a customer for Category 1 Programs prior to Takeda exercising its option, and for Category 2 Programs during the Category 2 Research Term. The Company identified the following material promises under the arrangement: (1) the non-exclusive, royalty-free research and development license for each Category 1 Program; (2) the research and development services for each Category 1 Program through completion of the first proof of mechanism study; (3) the exclusive option to license, co-develop and co-commercialize each Category 1 Program; (4) the right to exclusively license the Category 2 Programs; and (5) the research and preclinical development services of the Category 2 Programs through completion of IND-enabling studies. The research and development services for each Category 1 Program were determined to not be distinct from the research and development license and should therefore be combined into a single performance obligation for each Category 1 Program. The research and preclinical development services for the Category 2 Programs were determined to not be distinct from the exclusive licenses for the Category 2 Programs and should therefore be combined into a single performance obligation.

Additionally, the Company determined that the exclusive option for each Category 1 Program was priced at a discount and, as such, provide material rights to Takeda, representing three separate performance obligations. Based on these assessments, the Company identified seven performance obligations in the Takeda Collaboration Agreement: (1) research and development services through completion of the first proof of mechanism and non-exclusive research and development license for HD; (2) research and development services through completion of the first proof of mechanism and non-exclusive research and development license for ALS and FTD; (3) research and development services through completion of the first proof of mechanism and non-exclusive research and development license for SCA3; (4) the material right provided for the exclusive option to license, co-develop and co-commercialize HD; (5) the material right provided for the exclusive option to license, co-develop and co-commercialize ALS and FTD; (6) the material right provided for the exclusive option to license, co-develop and co-commercialize SCA3; and (7) the research and preclinical development services and right to exclusively license the Category 2 Programs.

At the outset of the arrangement, the transaction price included the $110.0 million upfront consideration received and the $60.0 million of committed research and preclinical funding for the Category 2 Programs. The Company determined that the Takeda Collaboration Agreement did not contain a significant financing component. The option exercise fees to license, co-develop and co-commercialize each Category 1 Program that may be received are excluded from the transaction price until each customer option is exercised. The potential milestone payments were excluded from the transaction price, as all milestone amounts were fully constrained at the inception of the Takeda Collaboration Agreement. The Company will reevaluate the transaction price at the end of each reporting period and, as uncertain events are resolved or other changes in circumstances occur, if necessary, will adjust its estimate of the transaction price.

Revenue associated with the research and development services for each Category 1 Program performance obligation is being recognized as the research and development services are provided using an input method, according to the costs incurred on each Category 1 Program and the total costs expected to be incurred to satisfy each Category 1 Program performance obligation. Revenue associated with the research and preclinical development services for the Category 2 Programs performance obligation is being recognized as the research and preclinical development services are provided using an input method, according to the costs incurred on Category 2 Programs and the total costs expected to be incurred to satisfy the performance obligation. The transfer of control for these performance obligations occurs over time and, in management’s judgment, this input method is the best measure of progress towards satisfying the performance obligations. The amount allocated to the material right for each Category 1 Program option will be recognized on the date that Takeda exercises each respective option, or immediately as each option expires unexercised. The amounts received that have not yet been recognized as revenue are recorded in deferred revenue on the Company’s consolidated balance sheet.

During the year ended December 31, 2018, the Company recognized revenue of $9.6 million in the Company’s consolidated statements of operations and comprehensive loss under the Takeda Collaboration Agreement. The aggregate amount of the transaction price allocated to the Company’s unsatisfied and partially unsatisfied performance obligations and recorded in deferred revenue at December 31, 2018 is $160.4 million, of which $94.1 million is included in current liabilities. The Company expects to recognize revenue for the portion of the deferred revenue that relates to the research and development services for each Category 1 Program and the Category 2 Programs as costs are incurred over the remaining research term. The Company expects to recognize revenue for the portion of the deferred revenue that relates to the material right for each Category 1 Program option upon Takeda’s exercise of such option, or immediately as each option expires unexercised. The aggregate amount of the transaction price included in accounts receivable at December 31, 2018 is $60.0 million, of which $10.0 million is included in current assets.

Share Capital

12 Months Ended

Dec. 31, 2018

Equity [Abstract]

Share Capital

6. SHARE CAPITAL

The following represents the historical ordinary share transactions of the Company from December 31, 2015 through December 31, 2018:

•

In May 2016, the Company granted 1,875,000 ordinary shares to Pfizer under the Pfizer Agreements (Note 5) at a purchase price of $16.00 per share, for an aggregate purchase price of $30.0 million.

•

In April 2017, the Company closed a follow-on underwritten public offering of 4,166,667 ordinary shares at a purchase price of $24.00 per share for gross proceeds of $100.0 million. The net proceeds from this issuance were $93.5 million after deducting underwriting discounts and commissions and other offering expenses.

•

In April 2018, the Company granted 1,096,892 ordinary shares to Takeda under the Takeda Collaboration Agreement (Note 5) at a purchase price of $54.70 per share, for an aggregate purchase price of $60.0 million.

Features of the Series A Preferred Shares and Ordinary Shares

The Series A preferred shares and ordinary shares have no par value and there is no concept of authorized share capital under Singapore law. The Series A preferred shares are not redeemable.

Voting

The holders of Series A preferred shares are not entitled to vote on any of the matters proposed to shareholders, other than as specified in the Company's Constitution. The holders of ordinary shares are entitled to one vote for each ordinary share held at all meetings of shareholders and written actions in lieu of meetings.

Dividends

All dividends, if any, shall be declared and paid pro rata according to the number of shares held by each member entitled to receive dividends. The Company’s board of directors may deduct from any dividend all sums of money presently payable by the member to the Company on account of calls.

Liquidation

In the event of a liquidation, dissolution or winding up of, or a return of capital by the Company, the ordinary shares will rank equally with the Series A preferred shares after the payment of the liquidation preference of an aggregate of approximately $10 thousand for Series A preferred shares.

Share-Based Compensation

12 Months Ended

Dec. 31, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Share-Based Compensation

7. SHARE-BASED COMPENSATION

In December 2014, the Company’s board of directors adopted the Wave Life Sciences Ltd. 2014 Equity Incentive Plan (the “2014 Plan”), and reserved 1,763,714 ordinary shares for issuance under this plan, which was increased to 5,064,544 in 2015, to 6,064,544 in 2017 and to 6,943,344 in 2018. The 2014 Plan authorizes the board of directors or a committee of the board to grant incentive share options, non-qualified share options, share appreciation rights, restricted awards, which include restricted shares and restricted share units (“RSUs”), and performance awards to eligible employees, consultants and directors of the Company. The company accounts for grants to its board of directors as grants to employees.

As of December 31, 2018, 1,545,151 ordinary shares remained available for future grant under the 2014 Plan.

Share option activity under the 2014 Plan is summarized as follows:

	Number of Shares			Weighted- Average Exercise Price		Weighted-Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value (in thousands)⁽¹⁾
Outstanding as of January 1, 2018		3,767,130		$	12.69
Granted		830,420		44.77
Exercised		(507,632	)	10.07
Forfeited or cancelled		(73,328	)	19.93
Outstanding as of December 31, 2018		4,016,590		$	19.47		7.24	$	93,520
Options exercisable as of December 31, 2018		2,451,584		$	10.11		6.51	$	78,280
Options unvested as of December 31, 2018		1,565,006		$	34.14		8.39	$	15,240

(1)	The aggregate intrinsic value of options is calculated as the difference between the exercise price of the share options and the fair value of the Company’s ordinary shares for those share options that had exercise prices lower than the fair value of the ordinary shares as of the end of the period.

Options generally vest over periods of one to four years, and options that are forfeited or cancelled are available to be granted again. The contractual life of options is generally five or ten years from the grant date.

The assumptions used in the Black-Scholes option pricing model to determine the fair value of share options granted to employees during the period were as follows:

	For the Year Ended December 31,
	2018	2017	2016
Risk-free interest rate	2.49% – 3.01%	1.49% – 2.23%	1.15% – 2.18%
Expected term (in years)	3.00 – 6.25	3.00 – 6.25	3.00 – 6.25
Expected volatility	64.17% – 70.90%	68.95% – 72.24%	60.89% – 68.76%
Expected dividend yield	0%	0%	0%

There were no options granted to non-employees in 2018, 2017 or 2016. The options that were granted to non-employees in 2015 were fully vested as of December 31, 2018.

RSU activity for the year ended December 31, 2018 is summarized as follows:

	RSUs			Average Grant Date Fair Value (in dollars per share)
Outstanding as of January 1, 2018		154,459			29.05
Granted		313,747			40.17
Vested		(38,594	)		26.30
Forfeited		(21,091	)		37.27
RSUs Outstanding at December 31, 2018		408,521		$	37.16

RSUs generally vest over periods of one to four years. RSUs that are forfeited are available to be granted again.

As of December 31, 2018, the unrecognized compensation cost related to outstanding options was $29.0 million for employees. The unrecognized compensation cost related to outstanding options for employees is expected to be recognized over a weighted-average period of approximately 2.8 years. For the years ended December 31, 2018, 2017 and 2016, the weighted-average grant date fair value per granted option was $27.90, $16.58 and $30.23, respectively. The aggregate fair value of options that vested during the years ended December 31, 2018, 2017 and 2016, was $9.3 million, $11.5 million and $4.7 million, respectively. The unrecognized compensation costs related to outstanding RSUs was $11.5 million as of December 31, 2018, and is expected to be recognized over a weighted-average period of approximately 2.89 years. The total fair value of RSUs vested during the years ended December 31, 2018 and 2017 was $1.8 million and $0.4 million, respectively. No RSUs vested during the year ended December 31, 2016.

Share-based compensation expense for the years ended December 31, 2018, 2017 and 2016 is classified as operating expenses in the consolidated statements of operations and comprehensive loss as follows:

	For the Year Ended December 31,
	2018		2017		2016
	(in thousands)
Research and development expenses	$	9,172	$	7,670	$	4,936
General and administrative expenses		6,424		4,473		1,911
Total share-based compensation expense	$	15,596	$	12,143	$	6,847

Of the total share-based compensation expense recorded for the years ended December 31, 2018, 2017 and 2016, $1.4 million, $2.9 million and $2.7 million, respectively, related to options granted to non-employees, all of which is included in research and development expenses on the consolidated statements of operations and comprehensive loss.

Commitments and Contingencies

12 Months Ended

Dec. 31, 2018

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

8. COMMITMENTS AND CONTINGENCIES

Lease Arrangements

The Company enters into lease arrangements for its facilities as well as certain equipment. A summary of the arrangements are as follows:

Operating Leases

On September 26, 2016, and as amended on December 31, 2016, the Company entered into a 10 year and 9 month lease, which includes two successive five year renewal options, for its facility in Lexington, Massachusetts, which the Company uses primarily for its cGMP manufacturing, as well as for additional laboratory and office space. Throughout the term of the lease, the Company is responsible for paying certain costs and expenses, in addition to the rent, as specified in the lease, including a proportionate share of applicable taxes, operating expenses and utilities. As required under the terms of the lease agreement, the Company has placed restricted cash of $2.6 million in a separate bank account at December 31, 2018 and 2017.

As of December 31, 2018, the Company has received the $11.4 million of tenant improvement allowances to which it was entitled per the lease for the Lexington, Massachusetts facility; of which $8.5 million, $2.8 million and $0.1 million were received during the years ended December 31, 2018, 2017 and 2016. The lease obligation related to the tenant improvement allowance is amortized over the period from the commencement of tenant improvement construction through to the end of the original lease term.

In April 2015, the Company entered into a lease agreement for an office and laboratory facility in Cambridge, Massachusetts, which commenced in October 2015 with a term of 7.5 years with a five-year renewal option to extend the lease. As required under the terms of the lease agreement, the Company has placed restricted cash of $1.0 million in a separate bank account at December 31, 2018 and 2017.

Future minimum lease payments under the Company’s non-cancelable operating leases as of December 31, 2018, are as follows:

For the Year Ended December 31,	Amount
	(in thousands)
2019	$	5,675
2020		5,846
2021		6,021
2022		6,201
2023		5,236
Thereafter		20,927
	$	49,906

The Company recorded rent expense of $4.9 million, $5.6 million and $1.5 million for the years ended December 31, 2018, 2017 and 2016, respectively.

Unasserted Claims

In the ordinary course of business, the Company may be subject to legal proceedings, claims and litigation as the Company operates in an industry susceptible to patent and other legal claims. The Company accounts for estimated losses with respect to legal proceedings and claims when such losses are probable and estimable. Legal costs associated with these matters are expensed when incurred. The Company is not currently a party to any material legal proceedings.

Net Loss Per Ordinary Share

12 Months Ended

Dec. 31, 2018

Earnings Per Share [Abstract]

Net Loss Per Ordinary Share

9. NET LOSS PER ORDINARY SHARE

Basic loss per share is computed by dividing net loss attributable to ordinary shareholders by the weighted-average number of ordinary shares outstanding:

	Year Ended December 31,
	2018			2017			2016
	(in thousands except share and per share data)
Numerator:
Net loss attributable to ordinary shareholders	$	(146,653	)	$	(101,980	)	$	(55,660	)
Denominator:
Weighted-average ordinary shares outstanding		28,970,404			26,513,382			22,800,628
Net loss per share, basic and diluted	$	(5.06	)	$	(3.85	)	$	(2.44	)

The Company’s potentially dilutive shares, which include outstanding share options to purchase ordinary shares and restricted share units, are considered to be ordinary share equivalents and are only included in the calculation of diluted net loss per share when their effect is dilutive.

The following potential ordinary shares, presented based on amounts outstanding at each period end, were excluded from the calculation of diluted net loss per share attributable to ordinary shareholders for the periods indicated because including them would have had an anti-dilutive effect:

	As of December 31,
	2018		2017
Options to purchase ordinary shares		4,016,590		3,767,130
RSUs		408,521		154,459
Series A preferred shares		3,901,348		3,901,348

Income Taxes

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Income Taxes

10. INCOME TAXES

During the years ended December 31, 2018, 2017 and 2016, the Company recorded a tax provision of $0.1 million, $0.8 million and $0.5 million, respectively. The 2018 tax provision was due to adjustments related to the filing of the Company’s 2017 tax returns with the relevant tax authorities. The 2017 tax provision was due to the Company’s establishment of a valuation allowance against the Company’s U.S. deferred tax assets, as well as income generated under research and management services arrangements between the Company’s U.S. and Singapore entities, which is taxed in the United States. The 2016 tax provision was primarily the result of income generated under research and management services arrangements between the Company’s U.S. and Singapore entities, which is taxed in the United States.

During the year ended December 31, 2018, the Company recorded no income tax benefit for the net operating losses incurred in Singapore, the United States, Japan and the United Kingdom, due to uncertainty regarding future taxable income in those jurisdictions. During the year ended December 31, 2017, the Company recorded no income tax benefit for the net operating losses incurred in Singapore and the United Kingdom, due to uncertainty regarding future taxable income in those jurisdictions. During the year ended December 31, 2016, the Company recorded no income tax benefit for the net operating losses incurred in Singapore and Japan, due to uncertainty regarding future taxable income in those jurisdictions. In May 2016, the Company established a wholly-owned subsidiary in Ireland, however no income tax expense or benefit has been recorded during the years ended December 31, 2018, 2017 or 2016.

The Tax Cuts and Jobs Act (the “Tax Act”) was enacted on December 22, 2017 and includes significant changes to the U.S. corporate tax system. Effective January 1, 2018, the Tax Act reduced the U.S. federal corporate tax rate from 35% to 21% and transitioned the U.S. federal tax system from a worldwide tax system to a territorial tax system. On December 22, 2017, the SEC issued Staff Accounting Bulletin 118 (“SAB 118”) that provides additional guidance allowing companies to apply a measurement period of up to twelve months to account for the impacts of the Tax Act in their financial statements. As of December 31, 2018, the Company has finalized its accounting for the impacts of the Tax Act. During the year ended December 31, 2018, the Company recognized a $0.2 million benefit related to the remeasurement of the Company’s deferred tax assets and liabilities, as a result of filing the Company’s 2017 U.S. tax return. This amount was included as a component of the Company’s provision for income taxes and was fully offset by a corresponding amount in the Company’s valuation allowance.

The components of the benefit (provision) for income taxes were as follows:

	Year Ended December 31,
	2018			2017			2016
	(in thousands)
Current benefit (provision) for income taxes:
Singapore	$	(4	)	$	199		$	—
Rest of world		(65	)		(133	)		(1,180	)
Total current benefit (provision) for income taxes	$	(69	)	$	66		$	(1,180	)
Deferred benefit (provision) for income taxes:
Singapore	$	—		$	(134	)	$	134
Rest of world		—			(774	)		564
Total deferred benefit (provision) for income taxes	$	—		$	(908	)	$	698
Total benefit (provision) for income taxes	$	(69	)	$	(842	)	$	(482	)

A reconciliation of the Singapore statutory income tax rate to the Company’s effective income tax rate is as follows:

	Year Ended December 31,
	2018			2017			2016
Singapore statutory income tax rate		17.0	%		17.0	%		17.0	%
Federal and state tax credits		6.6			5.7			3.1
Permanent differences		(0.3	)		(2.6	)		(0.9	)
Changes in reserves for uncertain tax positions		(2.3	)		(3.5	)		(3.6	)
Foreign rate differential		7.8			2.8			(0.1	)
Tax rate change		(0.3	)		(0.9	)		—
Other		0.2			0.4			(0.7	)
Change in deferred tax asset valuation allowance		(28.7	)		(19.7	)		(15.7	)
Effective income tax rate		—			(0.8	)%		(0.9	)%

The components of the Company’s deferred tax assets and liabilities as of December 31, 2018 and 2017 are as follows:

	December 31,
	2018			2017
	(in thousands)
Deferred tax assets:
Net operating loss carryforwards	$	58,661		$	28,913
Federal and state tax credits		13,783			4,522
Accrued expenses		4,276			1,903
Share-based compensation		3,811			1,921
Accumulated amortization		18,829			—
Other		1,267			176
Total deferred tax assets		100,627			37,435
Valuation allowance		(99,438	)		(36,069	)
Net deferred tax assets		1,189			1,366
Deferred tax liabilities:
Accumulated depreciation		(1,155	)		(1,366	)
Other		(34	)		—
Total deferred tax liabilities		(1,189	)		(1,366	)
Net deferred tax assets (liabilities)	$	—		$	—

A roll-forward of the valuation allowance for the years ended December 31, 2018 and 2017 is as follows:

	Year Ended December 31,
	2018		2017
	(in thousands)
Balance at beginning of year	$	36,069	$	15,999
Increase in valuation allowance		63,337		20,595
Reversal of valuation allowance		—		(598	)
Effect of foreign currency translation		32		73
Balance at end of year	$	99,438	$	36,069

As of December 31, 2018, the Company had federal net operating loss carryforwards in the United States of $82.6 million, $80.6 million of which may be available to offset future income tax liabilities indefinitely, while $2.0 million of carryforwards that were in existence as of December 31, 2017 may offset future income tax liabilities up through 2037. As of December 31, 2018 and 2017, the Company has U.S. federal research and development tax credit carryforwards of approximately $6.2 million and $2.8 million, respectively, available to offset future U.S. federal income taxes. As of December 31, 2018 and 2017, the Company has state research and development tax credit carryforwards of approximately $1.8 million and $1.1 million, respectively, available to offset future state income taxes. The U.S. federal and state research and development tax credits will begin to expire in 2031. As of December 31, 2018, the Company had a U.S. orphan drug credit carryforward of $5.0 million, which will begin to expire in 2037.

As of December 31, 2018 and 2017, the Company has net operating loss carryforwards in Japan of $2.9 million and $4.1 million, respectively, which may be available to offset future income tax liabilities and begin to expire in 2023.

As of December 31, 2018 and 2017, the Company has net operating loss carryforwards in Singapore of $161.3 million and $149.2 million, respectively, which may be available to offset future income tax liabilities and can be carried forward indefinitely.

As of December 31, 2018 and 2017, the Company has net operating loss carryforwards in the United Kingdom of $46.4 million and $10.5 million, which may be available to offset future income tax liabilities and can be carried forward indefinitely.

The Company has evaluated the positive and negative evidence bearing upon its ability to realize its deferred tax assets. As of December 31, 2018, management has considered the Company’s history of cumulative net losses incurred since inception and its lack of commercialization of any products or generation of any revenue from product sales since inception and has concluded that it is more likely than not that the Company will not realize the benefits of the deferred tax assets in all jurisdictions. Accordingly, a full valuation allowance has been established against those deferred tax assets as of December 31, 2018.

The valuation allowance increased by approximately $63.3 million in 2018, $20.1 million in 2017 and $8.5 million in 2016 primarily as a result of operating losses generated with no corresponding financial statement benefit. Additionally, as discussed in Note 2, the Company adopted ASU 2016-16 which resulted in an increase to the Company’s deferred tax assets, which was offset by a valuation allowance. The Company may release this valuation allowance when management determines that it is more-likely-than-not that the deferred tax assets will be realized. Any release of valuation allowance will be recorded as a tax benefit either increasing net income or decreasing net loss.

The Company’s reserves related to taxes and its accounting for uncertain tax positions are based on a determination of whether and how much of a tax benefit taken by the Company in its tax filings or positions is more-likely-than-not to be realized following resolution of any potential contingencies present related to the tax benefit.

A summary of activity in the Company’s unrecognized tax benefits is as follows:

	2018		2017		2016
	(in thousands)
Unrecognized tax benefit at the beginning of the year	$	6,207	$	2,343	$	1,280
Tax positions related to prior years		430		—		(1,066	)
Tax positions related to the current year		3,582		3,864		2,129
Unrecognized tax benefit at the end of the year	$	10,219	$	6,207	$	2,343

As of December 31, 2018, 2017 and 2016, the total amount of gross unrecognized tax benefits, which excludes interest and penalties, was $10.2 million, $6.2 million and $2.3 million, respectively. At December 31, 2018, $5.8 million of the net unrecognized tax benefits would affect the Company’s annual effective tax rate if recognized.

The Company does not expect to record any material reductions in the measurement of its unrecognized tax benefits within the next twelve months.

The Company’s policy is to record interest and penalties related to uncertain tax positions as part of its income tax provision. As of December 31, 2018 and 2017, the Company had recorded less than $0.1 million of interest or penalties related to uncertain tax positions.

The Company files income tax returns as prescribed by the tax laws of the jurisdictions in which it operates. In the normal course of business, the Company is subject to examination by various tax authorities in the United States, Japan, Singapore and the United Kingdom. There are currently no pending income tax examinations. Tax years from 2012 to the present are still open to examination in the United States, from 2008 to the present in Japan, from 2012 to the present in Singapore and from 2017 to the present in the United Kingdom. To the extent that the Company has tax attribute carryforwards, the tax years in which the attribute was generated may still be adjusted upon examination by the tax authorities to the extent utilized in a future period.

As of December 31, 2018 and 2017, $53.0 million and $48.8 million, respectively, of cash was held by the subsidiaries outside of Singapore. The Company does not provide for Singapore income tax or foreign withholding taxes on foreign unrepatriated earnings, as the Company intends to permanently reinvest undistributed earnings in its foreign subsidiaries. If the Company decides to change this assertion in the future to repatriate any additional foreign earnings, the Company may be required to accrue and pay taxes. Because of the complexity of Singapore and foreign tax rules applicable to the distribution of earnings from foreign subsidiaries to Singapore, the determination of the unrecognized deferred tax liability on these earnings is not practicable.

Utilization of the net operating loss carryforwards and research and development tax credit carryforwards in the United States may be subject to a substantial annual limitation under Section 382 of the Internal Revenue Code of 1986, as amended (the “Code”), due to ownership changes that have occurred previously or that could occur in the future. These ownership changes may limit the amount of carryforwards that can be utilized annually to offset future taxable income. In general, an ownership change, as defined by Section 382, results from transactions increasing the ownership of certain shareholders or public groups in the shares of a corporation by more than 50% over a three-year period. In 2018, the Company completed a study to assess whether an ownership change has occurred or whether there have been multiple ownership changes since its formation. The results of this study indicated that the Company experienced ownership changes as defined by Section 382 of the Code. Based on the results of the study, management has determined that the limitations will not have a material impact on the Company’s ability to utilize its net operating losses and research and development credit carryforwards to offset future tax liabilities. Should an ownership change have occurred after December 31, 2018 or occur in the future, the Company’s ability to utilize its net operating losses and research and development tax credit carryforwards may be limited.

Employee Benefit Plans

12 Months Ended

Dec. 31, 2018

Postemployment Benefits [Abstract]

Employee Benefit Plans

11. EMPLOYEE BENEFIT PLANS

The Company has a 401(k) retirement and savings plan (the “401(k) Plan”) covering employees of Wave USA. The 401(k) Plan allows employees to make contributions up to the maximum allowable amount set by the IRS. Under the 401(k) Plan, the Company may make discretionary contributions as approved by the board of directors. The Company made contributions of $0.7 million and $0.4 million in the years ended December 31, 2018 and 2017, respectively. The Company did not make contributions to the 401(k) Plan during the year ended December 31, 2016.

Related Parties

12 Months Ended

Dec. 31, 2018

Related Parties

12. RELATED PARTIES

The Company had the following related party transactions for the periods presented in the accompanying consolidated financial statements, which have not otherwise been discussed in these notes to the consolidated financial statements:

•

The Company held cash of $0.1 million in depository accounts with Kagoshima Bank, Ltd., an affiliate of one of the Company’s shareholders, Kagoshima Shinsangyo Sousei Investment Limited Partnership, as of December 31, 2017. These depository accounts were closed during the three months ended March 31, 2018.

•

Pursuant to the terms of various service agreements with Shin Nippon Biomedical Laboratories Ltd., one of the Company’s shareholders, and its affiliates (together “SNBL”), the Company paid SNBL $1.3 million, $0.5 million, and $0.4 million for the nine months ended September 30, 2018 and the years ended December 31, 2017 and 2016, respectively, for contract research services provided to the Company and its affiliates. SNBL ceased providing contract research services to the Company and its affiliates as of October 1, 2018.

•

In 2012, the Company entered into a consulting agreement for scientific services with Dr. Gregory L. Verdine, one of the Company’s founders and a member of the Company’s board of directors. The consulting agreement does not have a specific term and may be terminated by either party upon 14 days’ prior written notice. Pursuant to the consulting agreement, the Company pays Dr. Verdine approximately $13 thousand per month, plus reimbursement for certain expenses.

Selected Quarterly Financial Information (Unaudited)

12 Months Ended

Dec. 31, 2018

Quarterly Financial Information Disclosure [Abstract]

Selected Quarterly Financial Information (Unaudited)

13. SELECTED QUARTERLY FINANCIAL INFORMATION (UNAUDITED)

Selected quarterly results from operations for the years ended December 31, 2018 and 2017 are as follows:

	2018 Quarter Ended
	March 31			June 30			September 30			December 31
	(in thousands, except for per share data)
Revenues	$	1,422		$	4,879		$	4,493		$	3,620
Operating expenses		37,197			41,452			42,725			52,563
Loss from operations		(35,775	)		(36,573	)		(38,232	)		(48,943	)
Net loss		(35,241	)		(35,894	)		(37,631	)		(37,887	)
Basic and diluted net loss per ordinary share	$	(1.26	)	$	(1.23	)	$	(1.28	)	$	(1.29	)

	2017 Quarter Ended
	March 31			June 30			September 30			December 31
	(in thousands, except for per share data)
Revenues	$	383		$	1,097		$	1,315		$	1,098
Operating expenses		20,590			25,770			27,668			32,256
Loss from operations		(20,207	)		(24,673	)		(26,353	)		(31,158	)
Net loss		(21,096	)		(24,597	)		(25,494	)		(30,793	)
Basic and diluted net loss per ordinary share	$	(0.90	)	$	(0.91	)	$	(0.92	)	$	(1.11	)

Subsequent Events

12 Months Ended

Dec. 31, 2018

Subsequent Events [Abstract]

Subsequent Events

14. SUBSEQUENT EVENTS

On January 28, 2019, the Company closed a follow-on underwritten public offering of 3,950,000 ordinary shares for gross proceeds of $150.1 million, and on February 26, 2019, the Company closed on the sale of an additional 592,500 ordinary shares pursuant to the underwriters’ option (on the same terms and conditions as the initial closing) for gross proceeds of an additional $22.5 million (collectively, the “January 2019 Offering”). The net proceeds to the Company from the January 2019 Offering are expected to be approximately $161.6 million, after deducting underwriting discounts and commissions and estimated offering expenses.

Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Cash Equivalents

Principles of Consolidation

Use of Estimates

Segment Data

Foreign Currency Translation

Fair Value of Financial Instruments

Level 1—Unadjusted quoted prices in active markets that are accessible at the measurement date of identical, unrestricted assets.

Concentration of Credit Risk

Restricted Cash

Property and Equipment

Equipment, Furniture and Software	3-7 years
Leasehold Improvements	Shorter of asset life or lease term

Impairment of Long-Lived Assets

Revenue Recognition

For additional discussion of accounting for collaboration revenues, see Note 5.

Product Revenue

The Company has had no product revenue to date.

Research and Development Expenses

Refundable Tax Credits

Refundable tax credits are recorded as income and classified in other income (expense), net in the consolidated statements of operations and comprehensive loss.

Net Loss per Share

License Agreements and Patent Costs

Share-Based Compensation

•

Dividend Rate The expected dividend was assumed to be zero as the Company has never paid dividends and has no current plans to do so.

Income Taxes

The Company recognizes interest and penalties related to uncertain tax positions in the income tax provision on the consolidated statements of operations and comprehensive loss.

Recently Issued Accounting Pronouncements

Recently Adopted Accounting Pronouncements

Impact of Adoption

Consolidated Balance Sheets
	As of December 31, 2017
	As revised under ASC 606			As originally reported under ASC 605			Effect of change
	(in thousands)
Current portion of deferred revenue	$	1,275		$	2,705		$	(1,430	)
Deferred revenue, net of current portion		7,241			5,607			1,634
Accumulated deficit		(192,716	)		(192,512	)		(204	)

Consolidated Statements of Operations and Comprehensive Loss

	For the Year Ended December 31, 2017
	As revised under ASC 606			As originally reported under ASC 605			Effect of change
	(in thousands, except per share data)
Revenue	$	3,893		$	3,704		$	189
Loss from operations		(102,391	)		(102,580	)		189
Income tax benefit (provision)		(842	)		(708	)		(134	)
Net loss		(101,980	)		(102,035	)		55

	For the Year Ended December 31, 2016
	As revised under ASC 606			As originally reported under ASC 605			Effect of change
	(in thousands, except per share data)
Revenue	$	1,092		$	1,485		$	(393	)
Loss from operations		(55,720	)		(55,327	)		(393	)
Income tax benefit (provision)		(482	)		(616	)		134
Net loss		(55,660	)		(55,401	)		(259	)
Net loss per share attributable to ordinary shareholders—basic and diluted	$	(2.44	)	$	(2.43	)	$	(0.01	)

Consolidated Statement of Cash Flows
	For the Year Ended December 31, 2017
	As revised under ASC 606			As originally reported under ASC 605			Effect of change
	(in thousands)
Net loss	$	(101,980	)	$	(102,035	)	$	55
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Deferred income taxes		908			774			134
Changes in operating assets and liabilities:
Deferred revenue		(2,893	)		(2,704	)		(189	)

	For the Year Ended December 31, 2016
	As revised under ASC 606			As originally reported under ASC 605			Effect of change
	(in thousands)
Net loss	$	(55,660	)	$	(55,401	)	$	(259	)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Deferred income taxes		(698	)		(564	)		(134	)
Changes in operating assets and liabilities:
Deferred revenue		11,408			11,015			393

The most significant changes relate to the Company’s revenue recognition pattern for the Pfizer Collaboration Agreement (as defined in Note 5) and the accounting for milestone payments.

Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2018

Schedule of Estimated Useful Life of Assets

Depreciation is calculated on a straight-line basis over the following estimated useful lives of the assets:

Equipment, Furniture and Software	3-7 years
Leasehold Improvements	Shorter of asset life or lease term

ASC 606 [Member]

Summary of Result of Adopting ASC 606 on Financial Statement

Impact of Adoption

Consolidated Balance Sheets
	As of December 31, 2017
	As revised under ASC 606			As originally reported under ASC 605			Effect of change
	(in thousands)
Current portion of deferred revenue	$	1,275		$	2,705		$	(1,430	)
Deferred revenue, net of current portion		7,241			5,607			1,634
Accumulated deficit		(192,716	)		(192,512	)		(204	)

Consolidated Statements of Operations and Comprehensive Loss

	For the Year Ended December 31, 2017
	As revised under ASC 606			As originally reported under ASC 605			Effect of change
	(in thousands, except per share data)
Revenue	$	3,893		$	3,704		$	189
Loss from operations		(102,391	)		(102,580	)		189
Income tax benefit (provision)		(842	)		(708	)		(134	)
Net loss		(101,980	)		(102,035	)		55

	For the Year Ended December 31, 2016
	As revised under ASC 606			As originally reported under ASC 605			Effect of change
	(in thousands, except per share data)
Revenue	$	1,092		$	1,485		$	(393	)
Loss from operations		(55,720	)		(55,327	)		(393	)
Income tax benefit (provision)		(482	)		(616	)		134
Net loss		(55,660	)		(55,401	)		(259	)
Net loss per share attributable to ordinary shareholders—basic and diluted	$	(2.44	)	$	(2.43	)	$	(0.01	)

Consolidated Statement of Cash Flows
	For the Year Ended December 31, 2017
	As revised under ASC 606			As originally reported under ASC 605			Effect of change
	(in thousands)
Net loss	$	(101,980	)	$	(102,035	)	$	55
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Deferred income taxes		908			774			134
Changes in operating assets and liabilities:
Deferred revenue		(2,893	)		(2,704	)		(189	)

	For the Year Ended December 31, 2016
	As revised under ASC 606			As originally reported under ASC 605			Effect of change
	(in thousands)
Net loss	$	(55,660	)	$	(55,401	)	$	(259	)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Deferred income taxes		(698	)		(564	)		(134	)
Changes in operating assets and liabilities:
Deferred revenue		11,408			11,015			393

Property and Equipment, Net (Tables)

12 Months Ended

Dec. 31, 2018

Property Plant And Equipment [Abstract]

Schedule of Property and Equipment, Net

Property and equipment, net, consists of the following:

	December 31,
	2018			2017
	(in thousands)
Furniture and equipment	$	20,300		$	13,626
Software		435			108
Leasehold improvements		27,342			16,029
Fixed assets in progress		1,354			1,988
Total		49,431			31,751
Less accumulated depreciation		(9,500	)		(4,417	)
Property and equipment, net	$	39,931		$	27,334

Accrued Expenses and Other Current Liabilities (Tables)

12 Months Ended

Dec. 31, 2018

Payables And Accruals [Abstract]

Summary of Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consist of the following:

	December 31,
	2018		2017
	(in thousands)
Accrued compensation	$	7,507	$	5,461
Accrued expenses related to CROs and CMOs		5,502		3,281
Accrued expenses and other current liabilities		1,727		156
Total accrued expenses and other current liabilities	$	14,736	$	8,898

Share-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2018

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Summary of Share Option Activity

Share option activity under the 2014 Plan is summarized as follows:

	Number of Shares			Weighted- Average Exercise Price		Weighted-Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value (in thousands)⁽¹⁾
Outstanding as of January 1, 2018		3,767,130		$	12.69
Granted		830,420		44.77
Exercised		(507,632	)	10.07
Forfeited or cancelled		(73,328	)	19.93
Outstanding as of December 31, 2018		4,016,590		$	19.47		7.24	$	93,520
Options exercisable as of December 31, 2018		2,451,584		$	10.11		6.51	$	78,280
Options unvested as of December 31, 2018		1,565,006		$	34.14		8.39	$	15,240

(1)	The aggregate intrinsic value of options is calculated as the difference between the exercise price of the share options and the fair value of the Company’s ordinary shares for those share options that had exercise prices lower than the fair value of the ordinary shares as of the end of the period.

Summary of RSU Activity

RSU activity for the year ended December 31, 2018 is summarized as follows:

	RSUs			Average Grant Date Fair Value (in dollars per share)
Outstanding as of January 1, 2018		154,459			29.05
Granted		313,747			40.17
Vested		(38,594	)		26.30
Forfeited		(21,091	)		37.27
RSUs Outstanding at December 31, 2018		408,521		$	37.16

Summary of Share-based Compensation Expense Classified in Consolidated Statements of Operations and Comprehensive Loss

	For the Year Ended December 31,
	2018		2017		2016
	(in thousands)
Research and development expenses	$	9,172	$	7,670	$	4,936
General and administrative expenses		6,424		4,473		1,911
Total share-based compensation expense	$	15,596	$	12,143	$	6,847

Employee Stock Option [Member]

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Summary of Fair Value of Share Options Granted

The assumptions used in the Black-Scholes option pricing model to determine the fair value of share options granted to employees during the period were as follows:

	For the Year Ended December 31,
	2018	2017	2016
Risk-free interest rate	2.49% – 3.01%	1.49% – 2.23%	1.15% – 2.18%
Expected term (in years)	3.00 – 6.25	3.00 – 6.25	3.00 – 6.25
Expected volatility	64.17% – 70.90%	68.95% – 72.24%	60.89% – 68.76%
Expected dividend yield	0%	0%	0%

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2018

Commitments And Contingencies Disclosure [Abstract]

Future Minimum Lease Payments Under Operating Leases

Future minimum lease payments under the Company’s non-cancelable operating leases as of December 31, 2018, are as follows:

For the Year Ended December 31,	Amount
	(in thousands)
2019	$	5,675
2020		5,846
2021		6,021
2022		6,201
2023		5,236
Thereafter		20,927
	$	49,906

Net Loss Per Ordinary Share (Tables)

12 Months Ended

Dec. 31, 2018

Earnings Per Share [Abstract]

Summary of Basic and Diluted Net Loss Per Ordinary Share Outstanding

Basic loss per share is computed by dividing net loss attributable to ordinary shareholders by the weighted-average number of ordinary shares outstanding:

	Year Ended December 31,
	2018			2017			2016
	(in thousands except share and per share data)
Numerator:
Net loss attributable to ordinary shareholders	$	(146,653	)	$	(101,980	)	$	(55,660	)
Denominator:
Weighted-average ordinary shares outstanding		28,970,404			26,513,382			22,800,628
Net loss per share, basic and diluted	$	(5.06	)	$	(3.85	)	$	(2.44	)

Anti-Dilutive Shares Excluded from Calculation of Diluted Net Loss Per Ordinary Share

	As of December 31,
	2018		2017
Options to purchase ordinary shares		4,016,590		3,767,130
RSUs		408,521		154,459
Series A preferred shares		3,901,348		3,901,348

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2018

Deferred Components of Benefit (Provision) for Income Taxes

The components of the benefit (provision) for income taxes were as follows:

	Year Ended December 31,
	2018			2017			2016
	(in thousands)
Current benefit (provision) for income taxes:
Singapore	$	(4	)	$	199		$	—
Rest of world		(65	)		(133	)		(1,180	)
Total current benefit (provision) for income taxes	$	(69	)	$	66		$	(1,180	)
Deferred benefit (provision) for income taxes:
Singapore	$	—		$	(134	)	$	134
Rest of world		—			(774	)		564
Total deferred benefit (provision) for income taxes	$	—		$	(908	)	$	698
Total benefit (provision) for income taxes	$	(69	)	$	(842	)	$	(482	)

Components of Deferred Tax Assets and Liabilities

The components of the Company’s deferred tax assets and liabilities as of December 31, 2018 and 2017 are as follows:

	December 31,
	2018			2017
	(in thousands)
Deferred tax assets:
Net operating loss carryforwards	$	58,661		$	28,913
Federal and state tax credits		13,783			4,522
Accrued expenses		4,276			1,903
Share-based compensation		3,811			1,921
Accumulated amortization		18,829			—
Other		1,267			176
Total deferred tax assets		100,627			37,435
Valuation allowance		(99,438	)		(36,069	)
Net deferred tax assets		1,189			1,366
Deferred tax liabilities:
Accumulated depreciation		(1,155	)		(1,366	)
Other		(34	)		—
Total deferred tax liabilities		(1,189	)		(1,366	)
Net deferred tax assets (liabilities)	$	—		$	—

Summary of Valuation Allowance

A roll-forward of the valuation allowance for the years ended December 31, 2018 and 2017 is as follows:

	Year Ended December 31,
	2018		2017
	(in thousands)
Balance at beginning of year	$	36,069	$	15,999
Increase in valuation allowance		63,337		20,595
Reversal of valuation allowance		—		(598	)
Effect of foreign currency translation		32		73
Balance at end of year	$	99,438	$	36,069

Summary of Activity in Unrecognized Tax Benefits

A summary of activity in the Company’s unrecognized tax benefits is as follows:

	2018		2017		2016
	(in thousands)
Unrecognized tax benefit at the beginning of the year	$	6,207	$	2,343	$	1,280
Tax positions related to prior years		430		—		(1,066	)
Tax positions related to the current year		3,582		3,864		2,129
Unrecognized tax benefit at the end of the year	$	10,219	$	6,207	$	2,343

Singapore [Member]

Reconciliation of Statutory Income Tax Rate

A reconciliation of the Singapore statutory income tax rate to the Company’s effective income tax rate is as follows:

	Year Ended December 31,
	2018			2017			2016
Singapore statutory income tax rate		17.0	%		17.0	%		17.0	%
Federal and state tax credits		6.6			5.7			3.1
Permanent differences		(0.3	)		(2.6	)		(0.9	)
Changes in reserves for uncertain tax positions		(2.3	)		(3.5	)		(3.6	)
Foreign rate differential		7.8			2.8			(0.1	)
Tax rate change		(0.3	)		(0.9	)		—
Other		0.2			0.4			(0.7	)
Change in deferred tax asset valuation allowance		(28.7	)		(19.7	)		(15.7	)
Effective income tax rate		—			(0.8	)%		(0.9	)%

Selected Quarterly Financial Information (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2018

Quarterly Financial Information Disclosure [Abstract]

Schedule of Selected Quarterly Results from Operations

Selected quarterly results from operations for the years ended December 31, 2018 and 2017 are as follows:

	2018 Quarter Ended
	March 31			June 30			September 30			December 31
	(in thousands, except for per share data)
Revenues	$	1,422		$	4,879		$	4,493		$	3,620
Operating expenses		37,197			41,452			42,725			52,563
Loss from operations		(35,775	)		(36,573	)		(38,232	)		(48,943	)
Net loss		(35,241	)		(35,894	)		(37,631	)		(37,887	)
Basic and diluted net loss per ordinary share	$	(1.26	)	$	(1.23	)	$	(1.28	)	$	(1.29	)

	2017 Quarter Ended
	March 31			June 30			September 30			December 31
	(in thousands, except for per share data)
Revenues	$	383		$	1,097		$	1,315		$	1,098
Operating expenses		20,590			25,770			27,668			32,256
Loss from operations		(20,207	)		(24,673	)		(26,353	)		(31,158	)
Net loss		(21,096	)		(24,597	)		(25,494	)		(30,793	)
Basic and diluted net loss per ordinary share	$	(0.90	)	$	(0.91	)	$	(0.92	)	$	(1.11	)

Significant Accounting Policies - Additional Information (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) Segment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Significant Accounting Policies [Line Items]
Number of operating segment \| Segment	1
Cash, cash equivalents and restricted cash	$ 178,444	$ 146,113	$ 153,894	$ 162,275
Restricted cash	$ 3,625	$ 3,610
Expected dividend rate	0.00%
Description of Income tax benefit, likelihood of realized upon ultimate settlement	The amount of the benefit that may be recognized is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement.
Benefit related to remeasurement of deferred tax assets and liabilities as result of filing tax return	$ 200
Minimum [Member]
Significant Accounting Policies [Line Items]
Income tax examination, likelihood of settlement, percentage	50.00%
ASC 606 [Member]
Significant Accounting Policies [Line Items]
Revenue, practical expedient, incremental costs of obtaining contract	true
Accounting Standards Update No. 2016-16 [Member]
Significant Accounting Policies [Line Items]
Cumulative-effect adjustment to retained earnings	$ 400
Lexington, MA [Member]
Significant Accounting Policies [Line Items]
Restricted cash	2,600
Cambridge MA [Member]
Significant Accounting Policies [Line Items]
Restricted cash	$ 1,000

Significant Accounting Policies - Schedule of Estimated Useful Lives of Assets (Detail)	12 Months Ended
	Dec. 31, 2018
Equipment, Furniture and Software [Member] \| Minimum [Member]
Property Plant And Equipment [Line Items]
Property and equipment, estimated useful lives	3 years
Equipment, Furniture and Software [Member] \| Maximum [Member]
Property Plant And Equipment [Line Items]
Property and equipment, estimated useful lives	7 years
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Property and equipment, estimated useful lives	Shorter of asset life or lease term

Significant Accounting Policies - Additional Information (Detail 1)	Dec. 31, 2018
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2017-01-01
Significant Accounting Policies [Line Items]
Revenue recognition performance obligation, expected time satisfaction period	12 months

Significant Accounting Policies - Summary of Result of Adopting ASC 606 on Financial Statement - Consolidated Balance Sheets (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Condensed Balance Sheet Statements Captions [Line Items]
Current portion of deferred revenue	$ 100,945	$ 1,275
Deferred revenue, net of current portion	68,156	7,241
Accumulated deficit	$ (339,721)	(192,716)
As Originally Reported Under ASC 605 [Member] \| ASC 606 [Member]
Condensed Balance Sheet Statements Captions [Line Items]
Current portion of deferred revenue		2,705
Deferred revenue, net of current portion		5,607
Accumulated deficit		(192,512)
Effect of Change [Member] \| ASC 606 [Member]
Condensed Balance Sheet Statements Captions [Line Items]
Current portion of deferred revenue		(1,430)
Deferred revenue, net of current portion		1,634
Accumulated deficit		$ (204)

Significant Accounting Policies - Summary of Result of Adopting ASC 606 on Financial Statement - Consolidated Statements of Operations and Comprehensive Loss (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Condensed Income Statements Captions [Line Items]
Revenue	$ 3,620	$ 4,493	$ 4,879	$ 1,422	$ 1,098	$ 1,315	$ 1,097	$ 383	$ 14,414	$ 3,893	$ 1,092
Loss from operations	(48,943)	(38,232)	(36,573)	(35,775)	(31,158)	(26,353)	(24,673)	(20,207)	(159,523)	(102,391)	(55,720)
Income tax benefit (provision)									(69)	(842)	(482)
Net loss	$ (37,887)	$ (37,631)	$ (35,894)	$ (35,241)	$ (30,793)	$ (25,494)	$ (24,597)	$ (21,096)	$ (146,653)	$ (101,980)	$ (55,660)
Net loss per share attributable to ordinary shareholders—basic and diluted	$ (1.29)	$ (1.28)	$ (1.23)	$ (1.26)	$ (1.11)	$ (0.92)	$ (0.91)	$ (0.90)	$ (5.06)	$ (3.85)	$ (2.44)
As Originally Reported Under ASC 605 [Member] \| ASC 606 [Member]
Condensed Income Statements Captions [Line Items]
Revenue										$ 3,704	$ 1,485
Loss from operations										(102,580)	(55,327)
Income tax benefit (provision)										(708)	(616)
Net loss										(102,035)	$ (55,401)
Net loss per share attributable to ordinary shareholders—basic and diluted											$ (2.43)
Effect of Change [Member] \| ASC 606 [Member]
Condensed Income Statements Captions [Line Items]
Revenue										189	$ (393)
Loss from operations										189	(393)
Income tax benefit (provision)										(134)	134
Net loss										$ 55	$ (259)
Net loss per share attributable to ordinary shareholders—basic and diluted											$ (0.01)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Significant Accounting Policies - Summary of Result of Adopting ASC 606 on Financial Statement - Consolidated Statement of Cash Flows (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Condensed Cash Flow Statements Captions [Line Items]
Net loss	$ (146,653)	$ (101,980)	$ (55,660)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Deferred income taxes		908	(698)
Changes in operating assets and liabilities:
Deferred revenue	$ 160,585	(2,893)	11,408
As Originally Reported Under ASC 605 [Member] \| ASC 606 [Member]
Condensed Cash Flow Statements Captions [Line Items]
Net loss		(102,035)	(55,401)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Deferred income taxes		774	(564)
Changes in operating assets and liabilities:
Deferred revenue		(2,704)	11,015
Effect of Change [Member] \| ASC 606 [Member]
Condensed Cash Flow Statements Captions [Line Items]
Net loss		55	(259)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Deferred income taxes		134	(134)
Changes in operating assets and liabilities:
Deferred revenue		$ (189)	$ 393

- Definition

The consolidated profit or loss for the period, net of income taxes, including the portion attributable to the noncontrolling interest.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
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-Paragraph 1A
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-Name Accounting Standards Codification
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-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
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Reference 7: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
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Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
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Reference 10: http://www.xbrl.org/2003/role/exampleRef
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-URI http://asc.fasb.org/extlink&oid=84234705&loc=SL4591552-111686

+ Details

Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Property and equipment gross	$ 49,431	$ 31,751
Less accumulated depreciation	(9,500)	(4,417)
Property and equipment, net	39,931	27,334
Furniture and Equipment [Member]
Property Plant And Equipment [Line Items]
Property and equipment gross	20,300	13,626
Software [Member]
Property Plant And Equipment [Line Items]
Property and equipment gross	435	108
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Property and equipment gross	27,342	16,029
Fixed Assets in Progress [Member]
Property Plant And Equipment [Line Items]
Property and equipment gross	$ 1,354	$ 1,988

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property Plant And Equipment [Abstract]
Depreciation expense	$ 5,581	$ 2,155	$ 784

Accrued Expenses and Other Current Liabilities - Summary of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Accrued compensation	$ 7,507	$ 5,461
Accrued expenses related to CROs and CMOs	5,502	3,281
Accrued expenses and other current liabilities	1,727	156
Total accrued expenses and other current liabilities	$ 14,736	$ 8,898

Collaboration Agreements - Additional Information (Detail)			1 Months Ended			3 Months Ended								12 Months Ended			32 Months Ended
	Apr. 02, 2018	May 05, 2016 USD ($) $ / shares shares	Apr. 30, 2018 USD ($) Target $ / shares shares	Feb. 28, 2018 USD ($)	May 31, 2016 USD ($) Program $ / shares shares	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2018 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Equity investment aggregate purchase price														$ 60,000,000	$ 93,509,000	$ 30,000,000
Collaboration and license agreement, deferred revenue current						$ 100,945,000				$ 1,275,000				100,945,000	1,275,000		$ 100,945,000
Proceeds from issuance of ordinary shares, net of offering costs														60,000,000	93,509,000	30,000,000
Collaboration revenue recognized						3,620,000	$ 4,493,000	$ 4,879,000	$ 1,422,000	$ 1,098,000	$ 1,315,000	$ 1,097,000	$ 383,000	$ 14,414,000	3,893,000	1,092,000
Pfizer Inc. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of research programs, counterparty nomination \| Program					5
Research term, description														Under the Pfizer Collaboration Agreement, the parties agreed to collaborate during a four-year research term. During the research term, the Company is responsible to use its commercially reasonable efforts to advance up to five programs through to the selection of clinical candidates. At that stage, Pfizer may elect to license any of these Pfizer Programs exclusively and obtain exclusive rights to undertake the clinical development of the resulting clinical candidates into products and the potential commercialization of any such products thereafter.
Collaboration agreement, additional period after research program		2 years
Up-front consideration received		$ 10,000,000
Revenue recognized														$ 4,900,000	$ 3,900,000	$ 1,100,000	9,900,000
Collaboration and license agreement, deferred revenue						8,700,000								8,700,000			8,700,000
Collaboration and license agreement, deferred revenue current						6,800,000								$ 6,800,000			6,800,000
Remaining research term														16 months
Takeda [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Up-front consideration received			$ 110,000,000
Research, License and Option Agreement [Member] \| Pfizer Inc. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment under collaboration agreement					$ 10,000,000
Collaboration agreement refundable					$ 0
Shares issued under equity agreement \| shares		1,875,000			1,875,000
Equity investment aggregate purchase price		$ 30,000,000			$ 30,000,000
Purchase price per share \| $ / shares		$ 16.00			$ 16.00
Collaborative agreement research term					4 years
Collaboration agreement termination period		90 days
Collaboration And License Agreement [Member] \| Category One Programs [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of global costs and potential profits sharing ratio			50.00%
Collaboration And License Agreement [Member] \| Takeda [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment under collaboration agreement			$ 110,000,000
Collaboration agreement termination period	180 days
Collaboration and license agreement, deferred revenue						160,400,000								$ 160,400,000			160,400,000
Collaboration and license agreement, deferred revenue current						94,100,000								94,100,000			94,100,000
Collaboration and license agreement month and year			2018-02
Fund receivable for research and preclinical activities			$ 60,000,000
Research term under collaboration and license agreement			4 years
Collaboration agreement commencement date	Apr. 02, 2018
Collaboration revenue recognized														9,600,000
Collaboration and license agreement, accounts receivable						60,000,000								60,000,000			60,000,000
Collaboration and license agreement, accounts receivable current						$ 10,000,000								$ 10,000,000			$ 10,000,000
Collaboration And License Agreement [Member] \| Takeda [Member] \| Category One Programs [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of global costs and potential profits sharing ratio			50.00%
Collaboration And License Agreement [Member] \| Takeda [Member] \| Category Two Programs [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Fund receivable for research and preclinical activities			$ 60,000,000
Research term under collaboration and license agreement			4 years
Maximum targets for preclinical programs \| Target			6
Option to reach maximum targets for preclinical programs														any one time
Collaboration And License Agreement [Member] \| Takeda [Member] \| Minimum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, committed cash				$ 230,000,000
Collaboration and Share Purchase Agreements [Member] \| Takeda [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Shares issued under equity agreement \| shares			1,096,892
Equity investment aggregate purchase price			$ 60,000,000
Purchase price per share \| $ / shares			$ 54.70
Proceeds from issuance of ordinary shares, net of offering costs			$ 60,000,000
Equity investment agreement official closure month and year			2018-04

Share Capital - Additional Information (Detail) $ / shares in Units, $ in Thousands		1 Months Ended			12 Months Ended
	May 05, 2016 USD ($) $ / shares shares	Apr. 30, 2018 USD ($) $ / shares shares	Apr. 30, 2017 USD ($) $ / shares shares	May 31, 2016 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) Vote $ / shares shares	Dec. 31, 2017 USD ($) $ / shares	Dec. 31, 2016 USD ($)
Class of Stock [Line Items]
Equity investment aggregate purchase price \| $					$ 60,000	$ 93,509	$ 30,000
Net proceeds from sale of ordinary shares \| $					$ 60,000	$ 93,509	$ 30,000
Common stock, par value \| $ / shares					$ 0	$ 0
Ordinary share holder, voting right					One vote for each ordinary share
Voting rights, votes per share \| Vote					1
Preferred stock liquidation preference, per share \| $ / shares					$ 10
Singapore [Member]
Class of Stock [Line Items]
Common stock, par value \| $ / shares
Common stock, authorized \| shares					0
Series A Preferred Shares [Member] \| Singapore [Member]
Class of Stock [Line Items]
Preferred stock, par value \| $ / shares
Preferred stock, shares authorized \| shares					0
Follow-On Underwritten Public Offering [Member]
Class of Stock [Line Items]
Shares issued under equity agreement \| shares			4,166,667
Purchase price per share \| $ / shares			$ 24.00
Gross proceeds from sale of ordinary shares \| $			$ 100,000
Net proceeds from sale of ordinary shares \| $			$ 93,500
Research, License and Option Agreement [Member] \| Pfizer Inc. [Member]
Class of Stock [Line Items]
Shares issued under equity agreement \| shares	1,875,000			1,875,000
Purchase price per share \| $ / shares	$ 16.00			$ 16.00
Equity investment aggregate purchase price \| $	$ 30,000			$ 30,000
Collaboration and Share Purchase Agreements [Member] \| Takeda [Member]
Class of Stock [Line Items]
Shares issued under equity agreement \| shares		1,096,892
Equity investment aggregate purchase price \| $		$ 60,000
Net proceeds from sale of ordinary shares \| $		$ 60,000
Purchase price per share \| $ / shares		$ 54.70

Share-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation expense	$ 15,596	$ 12,143	$ 6,847
Research and Development Expenses [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation expense	$ 9,172	7,670	4,936
RSUs [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation weighted average recognition period	2 years 10 months 20 days
Unrecognized stock-based compensation expense	$ 11,500
Fair value of RSUs vested	$ 1,800	$ 400	$ 0
Minimum [Member] \| RSUs [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period	1 year
Maximum [Member] \| RSUs [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period	4 years
2014 Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Ordinary shares reserved for issuance					1,763,714
Number of shares authorized (in shares)	6,943,344	6,064,544		5,064,544
Ordinary shares available for future grant	1,545,151
Options to purchase common stock granted	830,420
Weighted average grant date fair value per granted option	$ 27.90	$ 16.58	$ 30.23
Aggregate fair value of options vested	$ 9,300	$ 11,500	$ 4,700
2014 Plan [Member] \| Non-Employees Stock Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options to purchase common stock granted	0	0	0
2014 Plan [Member] \| Non-Employees Stock Option [Member] \| Research and Development Expenses [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation expense	$ 1,400	$ 2,900	$ 2,700
2014 Plan [Member] \| Employee Stock Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation expense	$ 29,000
Unrecognized stock-based compensation weighted average recognition period	2 years 9 months 18 days
2014 Plan [Member] \| Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period	1 year
Contractual life of options	5 years
2014 Plan [Member] \| Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period	4 years
Contractual life of options	10 years

Share-Based Compensation - Summary of Share Option Activity (Detail) - 2014 Plan [Member] $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding, Number of Shares, Beginning Balance \| shares	3,767,130
Granted, Number of Shares \| shares	830,420
Exercised, Number of Shares \| shares	(507,632)
Forfeited or cancelled, Number of Shares \| shares	(73,328)
Outstanding, Number of Shares, Ending Balance \| shares	4,016,590
Options exercisable, Number of Shares \| shares	2,451,584
Options unvested, Number of Shares \| shares	1,565,006
Outstanding, Weighted Average Exercise Price, Beginning Balance \| $ / shares	$ 12.69
Granted, Weighted Average Exercise Price \| $ / shares	44.77
Exercised, Weighted Average Exercise Price \| $ / shares	10.07
Forfeited or cancelled, Weighted Average Exercise Price \| $ / shares	19.93
Outstanding, Weighted Average Exercise Price, Ending Balance \| $ / shares	19.47
Options exercisable, Weighted Average Exercise Price \| $ / shares	10.11
Options unvested, Weighted Average Exercise Price \| $ / shares	$ 34.14
Outstanding, Weighted Average Remaining Contractual Term	7 years 2 months 26 days
Options exercisable, Weighted Average Remaining Contractual Term	6 years 6 months 3 days
Options unvested, Weighted Average Remaining Contractual Term	8 years 4 months 20 days
Outstanding, Aggregate Intrinsic Value \| $	$ 93,520
Options exercisable, Aggregate Intrinsic Value \| $	78,280
Options unvested, Aggregate Intrinsic Value \| $	$ 15,240

Share-Based Compensation - Summary of Fair Value of Share Options Granted to Employees (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected dividend yield	0.00%
Employee Stock Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate, minimum	2.49%	1.49%	1.15%
Risk-free interest rate, maximum	3.01%	2.23%	2.18%
Expected volatility, minimum	64.17%	68.95%	60.89%
Expected volatility, maximum	70.90%	72.24%	68.76%
Expected dividend yield	0.00%	0.00%	0.00%
Employee Stock Option [Member] \| Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	3 years	3 years	3 years
Employee Stock Option [Member] \| Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	6 years 3 months	6 years 3 months	6 years 3 months

Share-Based Compensation - Summary of RSU Activity (Detail) - RSUs [Member]	12 Months Ended
	Dec. 31, 2018 $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding, Number of Shares, Beginning Balance \| shares	154,459
Granted, Number of Shares \| shares	313,747
Vested, Number of Shares \| shares	(38,594)
Forfeited, Number of Shares \| shares	(21,091)
Outstanding, Number of Shares, Ending Balance \| shares	408,521
Outstanding, Average Grant Date Fair Value, Beginning Balance \| $ / shares	$ 29.05
Granted, Average Grant Date Fair Value \| $ / shares	40.17
Vested, Average Grant Date Fair Value \| $ / shares	26.30
Forfeited, Average Grant Date Fair Value \| $ / shares	37.27
Outstanding, Average Grant Date Fair Value, Ending Balance \| $ / shares	$ 37.16

Share-Based Compensation - Summary of Share-based Compensation Expense Classified in Consolidated Statements of Operations and Comprehensive Loss (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Share-based compensation expense	$ 15,596	$ 12,143	$ 6,847
Research and Development Expenses [Member]
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Share-based compensation expense	9,172	7,670	4,936
General and Administrative Expenses [Member]
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Share-based compensation expense	$ 6,424	$ 4,473	$ 1,911

Commitments and Contingencies - Additional Information (Detail) - USD ($)	12 Months Ended			36 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Sep. 26, 2016	Apr. 30, 2015
Commitments And Contingencies [Line Items]
Restricted cash	$ 3,625,000	$ 3,610,000		$ 3,625,000
Lease rent expense	$ 4,900,000	5,600,000	$ 1,500,000
2016 Lexington, MA Lease [Member]
Commitments And Contingencies [Line Items]
Lease agreement term					10 years 9 months
Lease renewal term description	10 year and 9 month lease, which includes two successive five year renewal options
Restricted cash	$ 2,600,000	2,600,000		2,600,000
Lease extension period					5 years
Tenant improvement allowances received	8,500,000	2,800,000	$ 100,000	11,400,000
2015 Cambridge, MA Lease [Member]
Commitments And Contingencies [Line Items]
Lease agreement term						7 years 6 months
Restricted cash	$ 1,000,000	$ 1,000,000		$ 1,000,000
Lease extension period						5 years

Commitments and Contingencies - Future Minimum Lease Payments Under Operating Leases (Detail) $ in Thousands	Dec. 31, 2018 USD ($)
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2019	$ 5,675
2020	5,846
2021	6,021
2022	6,201
2023	5,236
Thereafter	20,927
Total	$ 49,906

Net Loss Per Ordinary Share - Summary of Basic and Diluted Net Loss Per Ordinary Share Outstanding (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Net loss attributable to ordinary shareholders									$ (146,653)	$ (101,980)	$ (55,660)
Denominator:
Weighted-average ordinary shares outstanding									28,970,404	26,513,382	22,800,628
Net loss per share, basic and diluted	$ (1.29)	$ (1.28)	$ (1.23)	$ (1.26)	$ (1.11)	$ (0.92)	$ (0.91)	$ (0.90)	$ (5.06)	$ (3.85)	$ (2.44)

Net Loss Per Ordinary Share - Anti-Dilutive Shares Excluded from Calculation of Diluted Net Loss Per Ordinary Share (Detail) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Options to Purchase Ordinary Shares [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive shares excluded from calculation of diluted net loss per share	4,016,590	3,767,130
RSUs [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive shares excluded from calculation of diluted net loss per share	408,521	154,459
Series A Preferred Shares [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive shares excluded from calculation of diluted net loss per share	3,901,348	3,901,348

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
Income tax benefit (provision)	$ (69,000)	$ (842,000)	$ (482,000)
U.S. federal corporate tax rate	21.00%	35.00%
Benefit related to remeasurement of deferred tax assets and liabilities as result of filing tax return	$ 200,000
Net operating loss carryovers	58,661,000	$ 28,913,000
Increase in valuation allowance	63,300,000	20,100,000	8,500,000
Gross unrecognized tax benefit	10,219,000	6,207,000	2,343,000	$ 1,280,000
Unrecognized tax benefit that could affect annual effective tax rate if recognized	5,800,000
Subsidiaries Outside of Singapore [Member]
Income Taxes [Line Items]
Cash held by subsidiaries	53,000,000	48,800,000
Maximum [Member]
Income Taxes [Line Items]
Interest and penalties related to uncertain tax positions	100,000	100,000
Federal [Member]
Income Taxes [Line Items]
Net operating loss carryforwards	82,600,000
Net operating loss carryforwards available to offset future income tax liabilities indefinitely	$ 80,600,000
Net operating loss carryforwards available to offset future income tax liabilities		2,000,000
Operating Loss Carry forward expiration year	2037
Research and development tax credit carryforward	2031
Federal [Member] \| Research and development
Income Taxes [Line Items]
Research and development tax credit carryforward	$ 6,200,000	2,800,000
Federal [Member] \| Orphan Drug Credits [Member]
Income Taxes [Line Items]
Research and development tax credit carryforward	$ 5,000,000
U.S.Orphan drug credit carryforward expiration year	2037
State [Member]
Income Taxes [Line Items]
Research and development tax credit carryforward	2031
State [Member] \| Research and development
Income Taxes [Line Items]
Research and development tax credit carryforward	$ 1,800,000	1,100,000
Singapore [Member]
Income Taxes [Line Items]
Income tax benefit (provision)	$ 0	$ 0	$ 0
U.S. federal corporate tax rate	17.00%	17.00%	17.00%
Net operating loss carryovers	$ 161,300,000	$ 149,200,000
Tax years open to examination	2012
United States [Member]
Income Taxes [Line Items]
Income tax benefit (provision)	$ 0
Tax years open to examination	2012
Japan [Member]
Income Taxes [Line Items]
Income tax benefit (provision)	$ 0		$ 0
Net operating loss carryovers	$ 2,900,000	4,100,000
Operating loss carryforwards expiration year	2023
Tax years open to examination	2008
UK [Member]
Income Taxes [Line Items]
Income tax benefit (provision)	$ 0	0
Net operating loss carryovers	$ 46,400,000	10,500,000
Tax years open to examination	2017
Ireland [Member]
Income Taxes [Line Items]
Income tax benefit (provision)	$ 0	$ 0	$ 0

Income Taxes - Deferred Components of Benefit (Provision) for Income Taxes (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current benefit (provision) for income taxes:
Total current benefit (provision) income taxes	$ (69,000)	$ 66,000	$ (1,180,000)
Deferred benefit (provision) for income taxes:
Total deferred benefit (provision) income taxes		(908,000)	698,000
Total benefit (provision) for income taxes	(69,000)	(842,000)	(482,000)
Singapore [Member]
Current benefit (provision) for income taxes:
Total current benefit (provision) income taxes	(4,000)	199,000
Deferred benefit (provision) for income taxes:
Total deferred benefit (provision) income taxes		(134,000)	134,000
Total benefit (provision) for income taxes	0	0	0
Rest Of World [Member]
Current benefit (provision) for income taxes:
Total current benefit (provision) income taxes	$ (65,000)	(133,000)	(1,180,000)
Deferred benefit (provision) for income taxes:
Total deferred benefit (provision) income taxes		$ (774,000)	$ 564,000

Income Taxes - Reconciliation of Statutory Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes [Line Items]
Singapore statutory income tax rate	21.00%	35.00%
Singapore [Member]
Income Taxes [Line Items]
Singapore statutory income tax rate	17.00%	17.00%	17.00%
Federal and state tax credits	6.60%	5.70%	3.10%
Permanent differences	(0.30%)	(2.60%)	(0.90%)
Changes in reserves for uncertain tax positions	(2.30%)	(3.50%)	(3.60%)
Foreign rate differential	7.80%	2.80%	(0.10%)
Tax rate change	(0.30%)	(0.90%)
Other	0.20%	0.40%	(0.70%)
Change in deferred tax asset valuation allowance	(28.70%)	(19.70%)	(15.70%)
Effective income tax rate		(0.80%)	(0.90%)

Income Taxes - Components of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Net operating loss carryforwards	$ 58,661	$ 28,913
Federal and state tax credits	13,783	4,522
Accrued expenses	4,276	1,903
Share-based compensation	3,811	1,921
Accumulated amortization	18,829
Other	1,267	176
Total deferred tax assets	100,627	37,435
Valuation allowance	(99,438)	(36,069)	$ (15,999)
Net deferred tax assets	1,189	1,366
Deferred tax liabilities:
Accumulated depreciation	(1,155)	(1,366)
Other	(34)
Total deferred tax liabilities	$ (1,189)	$ (1,366)

Income Taxes - Summary of Valuation Allowance (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Deferred Tax Assets Net Of Valuation Allowance [Abstract]
Balance at beginning of year	$ 36,069	$ 15,999
Increase in valuation allowance	63,337	20,595
Reversal of valuation allowance		(598)
Effect of foreign currency translation	32	73
Balance at end of year	$ 99,438	$ 36,069

Income Taxes - Summary of Activity in Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation Of Unrecognized Tax Benefits Excluding Amounts Pertaining To Examined Tax Returns Roll Forward
Unrecognized tax benefit at the beginning of the year	$ 6,207	$ 2,343	$ 1,280
Tax positions related to prior years	430
Tax positions related to prior years			(1,066)
Tax positions related to the current year	3,582	3,864	2,129
Unrecognized tax benefit at the end of the year	$ 10,219	$ 6,207	$ 2,343

Employee Benefit Plans - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
401(k) Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Discretionary contributions to defined contribution pension plan	$ 700,000	$ 400,000	$ 0

Related Parties - Additional Information (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Third-Party Investor [Member]
Related Party Transaction [Line Items]
Related party transaction amount			$ 100
Shin Nippon Biomedical Laboratories Ltd. (SNBL) [Member]
Related Party Transaction [Line Items]
Payment for contract research services	$ 1,300		$ 500	$ 400
Scientific Advisor [Member] \| Consulting Agreement [Member]
Related Party Transaction [Line Items]
Consulting agreement termination notice period		14 days
Consulting service expenses		$ 13

Selected Quarterly Financial Information (Unaudited) - Schedule of Selected Quarterly Results from Operations (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Selected Quarterly Financial Information [Abstract]
Revenue	$ 3,620	$ 4,493	$ 4,879	$ 1,422	$ 1,098	$ 1,315	$ 1,097	$ 383	$ 14,414	$ 3,893	$ 1,092
Operating expenses	52,563	42,725	41,452	37,197	32,256	27,668	25,770	20,590	173,937	106,284	56,812
Loss from operations	(48,943)	(38,232)	(36,573)	(35,775)	(31,158)	(26,353)	(24,673)	(20,207)	(159,523)	(102,391)	(55,720)
Net loss	$ (37,887)	$ (37,631)	$ (35,894)	$ (35,241)	$ (30,793)	$ (25,494)	$ (24,597)	$ (21,096)	$ (146,653)	$ (101,980)	$ (55,660)
Basic and diluted net loss per ordinary share	$ (1.29)	$ (1.28)	$ (1.23)	$ (1.26)	$ (1.11)	$ (0.92)	$ (0.91)	$ (0.90)	$ (5.06)	$ (3.85)	$ (2.44)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Subsequent Events - Additional Information (Detail) - USD ($) $ in Thousands			1 Months Ended	12 Months Ended
	Feb. 26, 2019	Jan. 28, 2019	Apr. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Subsequent Event [Line Items]
Net proceeds from sale of ordinary shares				$ 60,000	$ 93,509	$ 30,000
Follow-On Underwritten Public Offering [Member]
Subsequent Event [Line Items]
Shares issued under equity agreement			4,166,667
Gross proceeds from sale of ordinary shares			$ 100,000
Net proceeds from sale of ordinary shares			$ 93,500
Follow-On Underwritten Public Offering [Member] \| Subsequent Event [Member]
Subsequent Event [Line Items]
Shares issued under equity agreement	592,500	3,950,000
Gross proceeds from sale of ordinary shares	$ 22,500	$ 150,100
Net proceeds from sale of ordinary shares		$ 161,600