Wave Life Sciences (WVE) 10-K 12 Mar 18 2017 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2017	Mar. 01, 2018	Jun. 30, 2017
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	FY
Trading Symbol	WVE
Entity Registrant Name	WAVE LIFE SCIENCES LTD.
Entity Central Index Key	1,631,574
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	Accelerated Filer
Entity Common Stock, Shares Outstanding		27,992,244
Entity Public Float			$ 288,724,657

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 142,503	$ 150,293
Prepaid expenses and other current assets	7,985	1,483
Deferred tax assets		214
Total current assets	150,488	151,990
Long-term assets:
Property and equipment, net	27,334	8,607
Deferred tax assets		560
Restricted cash	3,610	3,601
Other assets	411	53
Total long-term assets	31,355	12,821
Total assets	181,843	164,811
Current liabilities:
Accounts payable	7,598	4,943
Accrued expenses and other current liabilities	8,898	4,434
Current portion of capital lease obligation	16	62
Current portion of deferred rent	60
Current portion of deferred revenue	2,705	2,705
Current portion of lease incentive obligation	344	11
Total current liabilities	19,621	12,155
Long-term liabilities:
Capital lease obligation, net of current portion		16
Deferred rent, net of current portion	4,214	680
Deferred revenue, net of current portion	5,607	8,311
Lease incentive obligation, net of current portion	3,094	116
Other liabilities	1,619	796
Total long-term liabilities	14,534	9,919
Total liabilities	34,155	22,074
Series A preferred shares, no par value; 3,901,348 shares issued and outstanding	7,874	7,874
Shareholders’ equity:
Ordinary shares, no par value; 27,829,079 and 23,502,169 shares issued and outstanding at December 31, 2017 and 2016, respectively	310,038	215,602
Additional paid-in capital	22,172	10,029
Accumulated other comprehensive income (loss)	116	(291)
Accumulated deficit	(192,512)	(90,477)
Total shareholders’ equity	139,814	134,863
Total liabilities, Series A preferred shares and shareholders’ equity	$ 181,843	$ 164,811

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
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	27,829,079	23,502,169
Common stock, shares outstanding	27,829,079	23,502,169

Consolidated Statements of Oper

Consolidated Statements of Operations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Revenue	$ 3,704	$ 1,485	$ 152
Operating expenses:
Research and development	79,309	40,818	9,057
General and administrative	26,975	15,994	10,393
Total operating expenses	106,284	56,812	19,450
Loss from operations	(102,580)	(55,327)	(19,298)
Other income (expense), net:
Dividend income	1,578	255
Interest income (expense), net	6	337	86
Other income (expense), net	(331)	(50)	56
Total other income (expense), net	1,253	542	142
Loss before income taxes	(101,327)	(54,785)	(19,156)
Income tax provision	(708)	(616)	(44)
Net loss	$ (102,035)	$ (55,401)	$ (19,200)
Net loss per share attributable to ordinary shareholders—basic and diluted	$ (3.85)	$ (2.43)	$ (1.83)
Weighted-average ordinary shares used in computing net loss per share attributable to ordinary shareholders—basic and diluted	26,513,382	22,800,628	10,501,455

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (102,035)	$ (55,401)	$ (19,200)
Other comprehensive loss:
Foreign currency translation	407	(332)	(15)
Comprehensive loss	$ (101,628)	$ (55,733)	$ (19,215)

Consolidated Statements of Seri

Consolidated Statements of Series A Preferred Shares and Shareholders' Equity - USD ($) $ in Thousands	Total	IPO [Member]	Series A Preferred Shares [Member]	Series B Preferred Shares [Member]	Ordinary Shares [Member]	Ordinary Shares [Member]IPO [Member]	Additional Paid-In-Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2014	$ 2,027		$ 7,874		$ 9,973			$ 56	$ (15,876)
Beginning balance, shares at Dec. 31, 2014			3,901,348		4,263,472
Issuance of ordinary shares, net of offering costs	11,631	$ 100,366			$ 11,631	$ 100,366
Issuance of ordinary shares, net of offering costs, shares					4,769,077	6,993,126
Share-based compensation	4,024				$ 842		$ 3,182
Share-based compensation, shares					190,856
Temporary equity, Issuance of Series B preferred, net of issuance costs				$ 62,532
Temporary equity, Issuance of Series B preferred, net of issuance costs, shares				5,334,892
Temporary equity, Reclassification of Series A preferred shares			$ 7,874
Temporary equity, Reclassification of Series A preferred shares, shares			3,901,348
Reclassification of Series A preferred shares, shares			(3,901,348)
Reclassification of Series A preferred shares	(7,874)		$ (7,874)
Temporary equity, Conversion of Series B preferred shares into ordinary shares upon initial public offering				$ (62,532)
Temporary equity, Conversion of Series B preferred shares into ordinary shares upon initial public offering, shares				(5,334,892)
Conversion of Series B preferred shares into ordinary shares upon initial public offering, shares					5,334,892
Conversion of Series B preferred shares into ordinary shares upon initial public offering	62,532				$ 62,532
Other comprehensive loss	(15)							(15)
Net loss	(19,200)								(19,200)
Ending balance at Dec. 31, 2015	153,491				$ 185,344		3,182	41	(35,076)
Ending balance, shares at Dec. 31, 2015					21,551,423
Temporary equity, Ending balance at Dec. 31, 2015			$ 7,874
Temporary equity, Ending 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 loss	(332)							(332)
Net loss	(55,401)								(55,401)
Ending balance at Dec. 31, 2016	$ 134,863				$ 215,602		10,029	(291)	(90,477)
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		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 loss	407							407
Net loss	(102,035)								(102,035)
Ending balance at Dec. 31, 2017	$ 139,814				$ 310,038		$ 22,172	$ 116	$ (192,512)
Ending balance, shares at Dec. 31, 2017					27,829,079
Temporary equity, Ending balance at Dec. 31, 2017			$ 7,874
Temporary equity, Ending balance, shares at Dec. 31, 2017	3,901,348		3,901,348

Consolidated Statements of Ser7

Consolidated Statements of Series A Preferred Shares and Shareholders' Equity (Parenthetical) - Ordinary Shares [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2015
Issuance costs	$ 491	$ 169
IPO [Member]
Issuance costs		$ 3,702

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities
Net loss	$ (102,035)	$ (55,401)	$ (19,200)
Adjustments to reconcile net loss to net cash flows used in operating activities:
Amortization of lease incentive obligation	(208)
Depreciation and amortization	2,155	784	594
Share-based compensation expense	12,143	6,847	4,024
Deferred rent	3,594	565	88
Loss on disposal of property and equipment	205
Deferred income taxes	774	(564)	36
Tax benefit related to share-based compensation		(310)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(6,502)	(1,337)	130
Other non-current assets	(358)
Accounts payable	3,892	3,369	1,648
Accrued expenses and other current liabilities	4,550	2,296	267
Deferred revenue	(2,704)	11,015	(152)
Other non-current liabilities	823	864	38
Net cash used in operating activities	(83,671)	(31,872)	(12,527)
Cash flows from investing activities
Increase in restricted cash	(9)	(2,599)	(1,055)
Proceeds from government grant reimbursements for property and equipment			3
Proceeds from the sale of property and equipment		4
Purchases of property and equipment	(18,887)	(5,567)	(1,857)
Net cash used in investing activities	(18,896)	(8,162)	(2,909)
Cash flows from financing activities
Proceeds from initial public offering, net of offering costs and underwriter commissions			101,444
Costs associated with initial public offering		(1,075)
Proceeds from issuance of ordinary shares, net of offering costs	93,509	30,000	11,631
Proceeds from issuance of Series B preferred shares, net of offering costs			62,532
Proceeds from government grant			112
Payments on capital lease obligation	(62)	(62)	(126)
Proceeds from the exercise of share options	927	258
Net cash provided by financing activities	94,374	29,121	175,593
Effect of foreign exchange rates on cash	403	(14)	15
Net increase (decrease) in cash and cash equivalents	(7,790)	(10,927)	160,172
Cash and cash equivalents at beginning of period	150,293	161,220	1,048
Cash and cash equivalents at end of period	142,503	150,293	161,220
Supplemental disclosure of cash flow information:
Deferred offering costs in accounts payable and accrued expenses at period end			1,075
Cash paid for interest	37	29
Cash paid for taxes, net of refunds	(11)	554
Equipment acquired for capital lease obligation			268
Property and equipment purchases in accounts payable and accrued expenses at period end	339	1,653	$ 306
Tenant improvements paid for by the landlord during the period	2,774	$ 128
Tenant improvements to be reimbursed by the landlord	$ 745

The Company

The Company	12 Months Ended
	Dec. 31, 2017
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 biotechnology company with an innovative and proprietary synthetic chemistry drug development platform that the Company is using to rationally design, develop and commercialize a broad pipeline of first-in-class or best-in-class nucleic acid therapeutic candidates for genetically defined diseases. Nucleic acid therapeutics are a growing and innovative class of drugs that have the potential to address diseases that have historically been difficult to treat with small molecule drugs or biologics. Nucleic acid therapeutics, or oligonucleotides, are comprised of a sequence of nucleotides that are linked together by a backbone of chemical bonds. The Company is initially developing oligonucleotides that target genetic defects to either reduce the expression of disease-promoting proteins or transform the production of dysfunctional mutant proteins into the production of functional proteins. 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 - - The Company’s primary activities since inception have been developing an innovative and proprietary synthetic chemistry drug development platform to design, develop and commercialize nucleic acid therapeutic programs, advancing the Company’s neurology franchise, expanding the Company’s research and development activities into additional therapeutic areas including ophthalmology and hepatic, advancing programs into the clinic, furthering clinical development of such clinical-stage programs, building the Company’s intellectual property, recruiting personnel 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 will be successfully completed, 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. In addition, the Company is dependent upon the services of its employees and consultants. The Company has never been profitable, and since its inception has incurred recurring operating losses. The Company expects to incur significant expenses and increasing operating losses for the foreseeable future. To date, the Company has primarily funded its operations through private placements of debt and equity securities, public offerings of its ordinary shares and collaborations with third parties. As of December 31, 2017, the Company has received an aggregate of approximately $323.2 million in net proceeds from these transactions. The Company received $89.3 million in net proceeds from private placements of its debt and equity securities, $100.4 million in net proceeds ($111.9 million gross proceeds) from its initial public offering, inclusive of the over-allotment exercise, $40.0 million under the Pfizer Agreements, including $10.0 million as an upfront payment under the Pfizer Collaboration Agreement and $30.0 million in the form of an equity investment, and $93.5 million in net proceeds ($100.0 million gross proceeds) from its April 2017 follow-on underwritten public offering. Basis of Presentation The Company has prepared the accompanying consolidated financial statements in conformity with generally accepted accounting principles in the United States of America (“U.S. GAAP”) and in U.S. dollars.

Significant Accounting Policies

Significant Accounting Policies	12 Months Ended
	Dec. 31, 2017
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 significant 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 Series A preferred shares on conversion of the related party notes payable, 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 period over which revenue is recognized under the Pfizer Collaboration Agreement (as defined in Note 5) 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 synthetic chemistry platform to develop and commercialize a broad pipeline of nucleic acid-based therapeutics. 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. Prior to 2017, Wave Japan had intercompany loans payable to Wave that were not expected to be settled in the foreseeable future which were therefore translated at the historical rate for the date of each capital transaction. In 2017, Wave Japan repaid the intercompany loans which resulted in a foreign exchange loss which is included in the consolidated statements of operations within other income (expense), net. Net unrealized gains and losses from foreign currency translation are reflected as accumulated other comprehensive (loss) income within Series A preferred shares and shareholders’ equity and consolidated statements of comprehensive loss. Gains and losses on foreign currency transactions are included in the consolidated statements of operations 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 and cash equivalents are Level 1 assets which are comprised of funds held in readily available checking and money market accounts. 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, 2017 and 2016, the Company had $3.6 million of restricted cash, of which $2.6 million related to the Lexington facility and the remaining $1.0 million related to the Cambridge facility. Property and Equipment Property and equipment, which consists of furniture and equipment and leasehold improvements are stated at cost less accumulated depreciation and amortization. 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 life of lease or useful life Depreciation and amortization 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. 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. Through December 31, 2017, the Company has not recognized any impairment charges. Revenue Recognition As of December 31, 2017, the Company’s only significant source of revenue is derived from the Pfizer Collaboration Agreement (as defined in Note 5), pursuant to which the Company and Pfizer (as defined in Note 5) have agreed to collaborate on the discovery, development and commercialization of stereopure oligonucleotide therapeutics for the Pfizer Programs (as defined in Note 5), each directed at a genetically-defined hepatic target selected by Pfizer. The Company entered into the Pfizer Collaboration Agreement in May 2016. The Company presents revenue from the Pfizer Collaboration Agreement under Financial Accounting Standards Board (“FASB”), Accounting Standards Codification (“ASC”) Topic 808, Collaborative Arrangements (“ASC 808”). In addition, the Company recognizes revenue in accordance with ASC Topic 605, Revenue Recognition (“ASC 605”). Accordingly, revenue is recognized for each unit of accounting when all of the following criteria are met: • persuasive evidence of an arrangement exists; • delivery has occurred or services have been rendered; • the seller’s price to the buyer is fixed or determinable; and • collectability is reasonably assured. Amounts received prior to satisfying the revenue recognition criteria 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. Pursuant to the accounting guidance in ASC 605‑25, the Company evaluates multiple‑element arrangements to determine (1) the deliverables included in the arrangement and (2) whether the individual deliverables represent separate units of accounting or whether they must be accounted for as a combined unit of accounting. This evaluation involves subjective determinations and requires the Company to make judgments about the individual deliverables and whether such deliverables are separable from the other aspects of the contractual relationship. Deliverables are considered separate units of accounting provided that the delivered item has value to the customer on a standalone basis and, if the arrangement includes a general right of return relative to the delivered item, delivery or performance of the undelivered item is considered probable and substantially in the Company’s control. In assessing whether an item has standalone value, the Company considers factors such as the research, development, manufacturing and commercialization capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. In addition, the Company considers whether the collaboration partner can use a deliverable for its intended purpose without the receipt of the remaining deliverable, whether the value of the deliverable is dependent on the undelivered item and whether there are other vendors that can provide the undelivered items. Under the Pfizer Collaboration Agreement, the Company and Pfizer agreed to collaborate on the discovery, development and commercialization of up to five Pfizer Programs, two of the five targets were declared upon initiation of the agreement in May 2016. The Pfizer Collaboration Agreement provides Pfizer with certain options to nominate up to three remaining programs and the Company is required to consider whether such options are substantive. Options are considered substantive if, at the inception of the arrangement, the Company is at risk as to whether the collaboration partner will choose to exercise the option. Factors that the Company considers in evaluating whether an option is substantive include whether the optional elements are essential to the functionality of other programs nominated, whether economic factors compel Pfizer to purchase the optional elements, the cost to exercise the option, the overall objective of the arrangement and, the benefit Pfizer might obtain from the arrangement without exercising the option. In August 2016, Pfizer nominated the third hepatic target under the Collaboration and pursuant to the terms of the Pfizer Collaboration Agreement, Pfizer had the option to nominate two additional targets by November 5, 2017. On November 5, 2017, the Company amended its Pfizer Collaboration Agreement to extend the target nomination period from November 5, 2017 to May 5, 2018. This amendment provides Pfizer with an additional six months to nominate the two remaining hepatic targets under the Pfizer Collaboration Agreement. When an option is considered substantive and there is no significant incremental discount, the option is not considered a deliverable in the arrangement and no consideration is allocated to it. Conversely, when an option is not considered substantive or it is considered substantive but is priced at an incremental discount, it is analyzed to determine if it should be combined with other deliverables in the arrangement. Options that are substantive and priced at a significant and incremental discount are further assessed to determine whether a portion of the upfront payment should be allocated to the option and other deliverables in the arrangement. At the inception of an arrangement that includes milestone payments, the Company evaluates whether each milestone is substantive and at risk to both parties on the basis of the contingent nature of the milestone. This evaluation includes an assessment of whether: (1) the consideration is commensurate with either the Company’s performance to achieve the milestone or the enhancement of the value of the delivered item(s) as a result of a specific outcome resulting from its performance to achieve the milestone, (2) the Aside from the program nomination payments, which relate to the options described above, the remaining milestone payments required under the Pfizer Collaboration Agreement are contingent upon the Company’s performance under the Pfizer Collaboration Agreement, including in certain instances, regulatory approval. The Company views these milestones as substantive and has excluded the amounts as allocable consideration at the outset of the arrangement. All commercial milestones will be accounted for in the same manner as royalties and recorded as revenue upon achievement of the milestone, assuming all other revenue recognition criteria are met. The Company recognizes arrangement consideration allocated to each unit of accounting when all of the revenue recognition criteria in ASC 605 are satisfied for that particular unit of accounting. In the event that a deliverable does not represent a separate unit of accounting, the Company recognizes revenue from the combined unit of accounting over the Company’s contractual or estimated performance period for the undelivered elements, which is typically the term of the Company’s research and development obligations. If there is no discernible pattern of performance or objectively measurable performance measures do not exist, then the Company recognizes revenue under the arrangement on a straight‑line basis over the period the Company is expected to complete its performance obligations. Conversely, if the pattern of performance in which the service is provided to the customer can be determined and objectively measurable performance measures exist, then the Company recognizes revenue under the arrangement using the proportional performance method. Revenue recognized is limited to the lesser of the cumulative amount of payments received or the cumulative amount of revenue earned, as determined using the straight‑line method or proportional performance method, as applicable, as of the period ending date. The Company has concluded that the deliverables under the Pfizer Collaboration Agreement relate primarily to the research and development required by the Company for each of the programs nominated by Pfizer. The remaining deliverables, including sample supplies provided by each party to fulfill its obligation as a licensee, participation on a joint steering committee to oversee the research and development activities, and regulatory responsibilities related to filings and obtaining approvals related to the products that may result from each program do not represent separate units of accounting based on their dependence on the research and development efforts. Because there is no discernible pattern of performance given the nature of the research and development efforts, the Company recognizes the allocated revenue for each deliverable under the Pfizer Collaboration Agreement on a straight ‑ Product Revenue The Company has had no product revenue to date. 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. 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 issuances of 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 in the same manner in which the award recipient’s payroll costs are classified or in which the award recipients’ service payments are classified. Prior to the Company’s initial public offering (“IPO”) in November 2015, the fair value of the ordinary shares underlying its share-based awards was estimated on each grant date by the board of directors. The board of directors determined the estimated per share fair value of the Company’s ordinary shares at various dates considering contemporaneous and retrospective valuations performed in accordance with the guidance outlined in the American Institute of Certified Public Accountants Practice Aid, Valuation of Privately-Held Company Equity Securities Issued as Compensation The fair value of each share option grant was determined using the methods and assumptions discussed below. Each of these inputs is subjective and generally requires significant judgment and estimation by management. • Fair Value of Ordinary Shares . As discussed above, prior to the Company’s IPO, the fair value of the Company’s ordinary shares underlying the Company’s share options was historically determined by the board of directors. Because prior to the Company’s IPO, there was no public market for the Company’s ordinary shares, the board of directors determined the fair value of the Company’s ordinary shares at the time of grant of the option by considering a number of objective and subjective factors, including valuations of comparable companies, sales of its shares to unrelated third parties, operating and financial performance and general and industry specific economic outlook. Following the completion of the Company’s IPO, 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 taxing 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. 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 the related ongoing results of the comparable companies. Recently Issued Accounting Pronouncements In May 2014, the FASB issued Accounting Standards Update (“ASU”) No. 2014-09, Revenue from Contracts with Customers (Topic 606) (“ASU 2014-09”), which supersedes the revenue recognition requirements in ASC 605-25, Multiple-Element Arrangements Under ASC 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which 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 steps: (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. ASC 606 also impacts certain other areas, such as the accounting for costs to obtain or fulfill a contract. The standard also requires disclosure of the nature, amount, timing, and uncertainty of revenue and cash flows arising from contracts with customers. The Company is assessing but has not yet completed its assessment of the impact of the adoption of this standard on its consolidated financial statements. Therefore, the Company does not know and cannot reasonably estimate the impact that adoption of ASC 606 is expected to have on the consolidated financial statements. Currently, the Company anticipates a potential impact on the revenue recognition method used to recognize revenue for the identified performance obligations under the Pfizer Collaboration Agreement as well as the recognition of milestone revenue prior to achievement. The expected impact is further described below. Estimated impacts from the adoption of this standard could differ upon the final adoption and implementation of the standard. With respect to the Pfizer Collaboration Agreement, the Company currently expects the five performance obligations identified under the provisions of ASC 606 will be consistent with the five units of accounting identified under the provisions of ASC 605. However, as previously described, it currently expects that the timing and pattern of revenue recognition under step (v) above will differ from the pattern of revenue recognition under ASC 605. Under ASC 606, the revenues will be recognized over time. As of December 31, 2017, the Company had recognized $5.2 million of revenue under the Pfizer Collaboration Agreement. Deferred revenue related to the Pfizer Collaboration Agreement amounted to $8.3 million as of December 31, 2017, of which $2.7 million is included in current liabilities. The Company expects a change in the timing and pattern of revenue recognition upon adoption of ASC 606 to impact the Company’s revenue, deferred revenue and net loss. The Company expects the accounting for contingent milestone payments under its collaboration agreements to change under ASC 606. ASC 606 does not contain guidance specific to milestone payments, thereby requiring contingent milestone payments to be considered in accordance with the overall model of ASC 606. Revenue from contingent milestone payments may be recognized earlier under ASC 606 than under ASC 605, based on an assessment of the probability of achievement of the milestone event and the likelihood of a significant reversal of such milestone revenue at each reporting date. This assessment may result in the recognition of revenue related to a contingent milestone payment before the milestone event has been achieved. ASC 606 requires more robust disclosures than required by previous guidance, including disclosures related to disaggregation of revenue into appropriate categories, performance obligations, the judgments made in revenue recognition determinations, adjustments to revenue which relate to activities from previous quarters or years, any significant reversals of revenue, and costs to obtain or fulfill contracts. In connection with the adoption of these standards, the Company is implementing several new internal controls, including controls to monitor the probability of achievement of contingent milestone payments and the timing and pattern of performance of the performance obligation. In February 2016, the FASB issued Accounting Standards Update No. 2016-02, Leases , 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”), which requires an entity 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. These amendments should be applied on a modified retrospective basis through a cumulative-effect adjustment directly to retained earnings at the beginning of the period adopted. The Company will adopt ASU 2016-16 in the first quarter of 2018 and the Company estimates that there will be a cumulative-effect increase of approximately $0.4 million to the Company’s accumulated deficit. In November 2016, the FASB issued Accounting Standards Update No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash (“ASU 2016-18”). The ASU requires an entity to explain the changes in the total of cash, cash equivalents, restricted cash, and restricted cash equivalents on the statement of cash flows and to provide a reconciliation of the totals in that statement to the related captions in the balance sheet when the cash, cash equivalents, restricted cash, and restricted cash equivalents are presented in more than one line item on the balance sheet. This ASU is effective for annual and interim periods beginning after December 15, 2017, and is required to be adopted using a retrospective approach, with early adoption permitted. The Company is currently evaluating the potential impact that the adoption of ASU 2016-18 may have on its consolidated financial statements. Other accounting standards that have been issued or proposed by the FASB or other standards-setting bodies that do not require adoption until a future date are not expected to have a material impact on the Company’s consolidated financial statements upon adoption. Recently Adopted Accounting Pronouncements In November 2015, the FASB issued Accounting Standards Update No. 2015-17, Balance Sheet Classification of Deferred Taxes (“ASU 2015-17”),

Property and Equipment, Net

Property and Equipment, Net	12 Months Ended
	Dec. 31, 2017
Property Plant And Equipment [Abstract]
Property and Equipment, Net	3. PROPERTY AND EQUIPMENT, NET Property and equipment, net, consists of the following December 31, 2017 2016 (in thousands) Furniture and equipment $ 13,626 $ 7,231 Software 108 43 Leasehold improvements 16,029 1,964 Fixed assets in progress 1,988 1,863 Total 31,751 11,101 Less accumulated depreciation and amortization (4,417 ) (2,494 ) Property and equipment, net $ 27,334 $ 8,607 Leasehold improvements made during the years ended December 31, 2017 and 2016 consisted of costs related to the Company’s leased facilities in Cambridge, Massachusetts and Lexington, Massachusetts. Depreciation and amortization expense was $2.2 million, $0.8 million and $0.6 million for the years ended December 31, 2017, 2016 and 2015, respectively.

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	12 Months Ended
	Dec. 31, 2017
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, 2017 2016 (in thousands) Accrued compensation $ 5,428 $ 2,480 Accrued professional fees 3,281 417 Accrued vacation 33 589 Other current liabilities 156 948 Total accrued expenses and other current liabilities $ 8,898 $ 4,434

Pfizer Collaboration and Share

Pfizer Collaboration and Share Purchase Agreement	12 Months Ended
	Dec. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Pfizer Collaboration and Share Purchase Agreement	5. PFIZER COLLABORATION AND SHARE PURCHASE AGREEMENT On May 5, 2016, the Company entered into a Research, License and Option Agreement (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 the 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 to have 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 receives a non-exclusive, royalty-bearing sublicenseable 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. Pfizer nominated two hepatic targets upon entry into the Pfizer Collaboration in May 2016. In August 2016, Pfizer nominated the third hepatic target under the Pfizer Collaboration for which the Company received a $2.5 million milestone payment in 2016. On November 5, 2017, the Company amended its Pfizer Collaboration Agreement to extend the target nomination period from November 5, 2017 to May 5, 2018. This amendment provides Pfizer with an additional six months to nominate the two remaining hepatic targets under the Pfizer Collaboration Agreement. The Company has determined that the options held by Pfizer under the Pfizer Collaboration Agreement are substantive and priced at a significant incremental discount. Accordingly, $3.0 million of the upfront payment was allocated to the options to nominate the three remaining targets upon inception. The amount allocated to the three options will be recognized as the research and development services are provided commencing from the date that Pfizer exercises each respective option, or immediately as each option expires unexercised. The portion of the upfront payment allocated to the initial two targets was $7.0 million and will be recognized as the research and development services are provided from the inception of the arrangement. Subsequently, in 2016, Pfizer exercised its option to nominate a third program. The Company will recognize $3.5 million of revenue (which is comprised of $1.0 million allocated to the option at inception of the arrangement and $2.5 million paid by Pfizer at the time of exercising the option) as the research and development services are provided. In November 2017, the Company achieved a milestone under the Pfizer Collaboration Agreement, the revenue related to this milestone was recognized in full during the year ended December 31, 2017. 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. 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. During the year ended December 31, 2017, the Company recognized revenue of $3.7 million under the Pfizer Collaboration Agreement. Deferred revenue amounted to $8.3 million as of December 31, 2017, of which $2.7 million is included in current liabilities.

Share Capital

Share Capital	12 Months Ended
	Dec. 31, 2017
Equity [Abstract]
Share Capital	6. SHARE CAPITAL Ordinary Shares The following represents the historical ordinary share transactions of the Company from December 31, 2013 through December 31, 2017: • In February 2014, the Company issued 2,263,291 ordinary shares to a third-party investor at $2.47 per share for net proceeds of $5.6 million. In connection with this financing, holders of $9.6 million of related party notes payable agreed to convert such notes into 2,365,139 Series A preferred shares and 1,515,596 ordinary shares. • In January 2015, the Company issued 4,769,077 ordinary shares to a third-party investor and an existing investor at $2.47 per share for net proceeds of $11.6 million. • In March 2015, the Company granted 190,856 fully-vested ordinary shares to an executive of the Company. • In November 2015, the Company completed an initial public offering of its ordinary shares, in which the Company issued and sold 6,375,000 ordinary shares at a price to the public of $16.00 per share. In December 2015, the Company issued an additional 618,126 ordinary shares at a price of $16.00 per share pursuant to a partial exercise of the underwriters’ over-allotment option. The aggregate net proceeds to the Company from the initial public offering, inclusive of the over-allotment exercise, were $100.4 million after deducting underwriting discounts and commissions and estimated offering expenses payable by the Company. In connection with this financing, the Company’s 5,334,892 Series B preferred shares automatically converted into 5,334,892 of the Company’s ordinary shares. • 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 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 estimated offering expenses Features of the Ordinary Shares The ordinary shares have no par value and there is no concept of authorized share capital under Singapore law. The rights, preferences, and privileges of ordinary shares are as follows: New Share Offering Prior to the closing of the Company’s initial public offering, any new ordinary shares or securities convertible into ordinary shares were required to be offered in the first instance to all the then holders of any class of shares, other than the Series A preferred shares, prior to issuance and each shareholder had the right of pre-emption with respect to any issuance of new ordinary shares or securities convertible into ordinary shares. This right of pre-emption did not apply to shares sold in the Company’s initial public offering and terminated immediately prior to the closing of the Company’s initial public offering. Voting 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 $10.00 for Series A preferred shares. Series A Preferred Shares The following represent the Series A preferred share transactions of the Company from December 31, 2013 through December 31, 2017: • In February 2014, holders of $9.6 million of related party notes payable agreed to convert such notes into 2,365,139 Series A preferred shares and 1,515,596 ordinary shares. • In connection with the private placement of Series B preferred shares on August 14, 2015, holders of the Company’s preference shares agreed to rename the existing “preference shares” as “Series A preferred shares.” In addition, as further described below, the terms of the Series A preferred shares were amended to remove their right of first refusal and to provide for their right to convert on a one-for-one basis into an aggregate of 3,901,348 ordinary shares at any time at the election of the holder. The rights of the Series A preferred shares are identical to the ordinary shares except that the Series A preferred shares have: (1) no voting rights other than in limited circumstances, (2) the right to a non-cumulative dividend if and when declared by the Company’s board of directors and (3) the right to convert the Series A preferred shares at any time on a one-for-one basis into ordinary shares at the discretion of the holder. The Company’s shareholders, including holders of Series A preferred shares, entered into an investors’ rights agreement and a voting agreement with the Company in connection with the private placement. Pursuant to the terms of the voting agreement, which terminated in connection with the Company’s IPO, investors who held at least 1,212,477 shares of registerable securities, including holders of Series A preferred shares and Series B preferred shares, had a right to purchase certain new securities offered by the Company. Additionally, in the event of the sale of 50% or more of the voting power of the Company or a deemed liquidation event, if the holders of at least a majority of the ordinary shares and the holders of 56% of the Series B preferred shares had voted for a sale of the Company, they had the right to force the other shareholders, including the holders of Series A preferred shares, to agree to such a sale. • In September 2015, the terms of the Series A preferred shares were further amended to provide that, upon the mandatory conversion of Series B preferred shares, which occurred on the completion of the initial public offering, the existing right of Series A preferred shares to a non-cumulative dividend if and when declared by our board of directors ceased and was replaced by a liquidation preference consisting of $0.002 per Series A preferred share, or an aggregate of $10.00 based on the number of Series A preferred shares outstanding at the date of the amendment. The Company has accounted for the September 2015 amendment to the Series A preferred shares as a modification of the preferred shares based on upon a qualitative assessment of the amendment. The Company has not adjusted the carrying value of the Series A preferred shares since the fair value of the Series A preferred shares immediately prior and subsequent to the modification date resulted in an immaterial change in fair value. The addition of the liquidation preference to the Series A preferred shares, however, resulted in the reclassification of the Series A preferred shares from permanent shareholders’ equity to temporary shareholders’ equity since the holders of the Series A preferred shares are entitled to a liquidation preference upon a deemed liquidation event, which is outside the control of the Company. In the event a deemed liquidation event were to occur, the Company would adjust the carrying value of the Series A preferred shares to their liquidation value, which amounts to $10.00 in the aggregate. The Series A preferred 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. Series B Preferred Shares Converted in Connection with Initial Public Offering The following represents the historical Series B preferred share transactions of the Company from January 1, 2015 through the completion of our initial public offering: • On August 14, 2015, the Company issued an aggregate of 5,334,892 Series B preferred shares at a purchase price of $12.37 per share to certain third-party investors for $62.5 million of net proceeds. • Upon the completion of the initial public offering on November 16, 2015, all of the outstanding Series B preferred shares of the Company automatically converted into 5,334,892 of the Company’s ordinary shares. Prior to the conversion of the Series B preferred shares into ordinary shares, the Series B preferred shares had a liquidation preference over the Series A preferred shareholders and ordinary shareholders equal to the original per share amount paid of $12.37 per share, plus any declared plus unpaid dividends, if any. Additionally, the holders of Series B preferred shares were entitled to voting rights, however, the Series B preferred shareholders were not entitled to any preferential dividends and their shares were not redeemable.

Share-Based Compensation

Share-Based Compensation	12 Months Ended
	Dec. 31, 2017
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 and to 6,064,544 in 2017 As of December 31, 2017, 1,716,110 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, 2017 3,577,766 $ 10.58 Granted 522,750 26.33 Exercised (137,493 ) 6.74 Forfeited or cancelled (195,893 ) 14.71 Outstanding as of December 31, 2017 3,767,130 $ 12.69 7.81 $ 84,675 Options exercisable as of December 31, 2017 2,257,455 $ 7.64 7.43 $ 62,060 Options unvested as of December 31, 2017 1,509,675 $ 20.23 8.37 $ 22,614 (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 a period of three or 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. Share-based compensation expense related to options is included in research and development expenses or general and administrative expenses on the consolidated statements of operations. 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, 2017 2016 2015 Risk-free interest rate 1.49% – 2.23% 1.15% – 2.18% 1.56% - 1.89% Expected term (in years) 3.00 – 6.25 3.00 – 6.25 5.52 - 6.12 Expected volatility 68.95% – 72.24% 60.89% – 68.76% 62.14% - 71.02% Expected dividend yield 0% 0% 0% The assumptions used in the Black-Scholes option pricing model to determine the fair value of share options granted to non-employees during the period were as follows: Year Ended December 31, 2015 Risk-free interest rate 2.06% - 2.35% Expected term (in years) 9.19 - 10.00 Expected volatility 62.65% - 69.80% Expected dividend yield 0% There were no options granted to non-employees in 2017 or 2016. RSU activity for the years ended December 31, 2017 and 2016 is summarized as follows: RSUs Average Grant Date Fair Value (in dollars per share) Outstanding as of January 1, 2017 22,750 21.69 Granted 170,859 29.05 Vested (22,750 ) 21.69 Forfeited (16,400 ) 29.05 RSUs Outstanding at December 31, 2017 154,459 $ 29.05 There were no RSUs granted in 2015. The RSUs granted in 2016 fully vested upon the first anniversary of the grant date and the RSUs granted in 2017 vest annually over a period four years. RSUs that are forfeited are available to be granted again. Share-based compensation expense related to the RSUs is included in research and development expenses or general and administrative expenses on the consolidated statements of operations. As of December 31, 2017, the unrecognized compensation cost related to outstanding options was $16.9 million for employees and $1.0 million for non-employees. The unrecognized compensation cost related to outstanding options for employees and non-employees is expected to be recognized over a weighted-average period of approximately 2.6 years. For the years ended December 31, 2017 and 2016, the weighted-average grant date fair value per granted option was $16.58 and $30.23, respectively. The aggregate fair value of options that vested during the year ended December 31, 2017 was $11.5 million. The unrecognized compensation costs related to outstanding RSUs was $3.5 million as of December 31, 2017, and is expected to be recognized over a weighted-average period of approximately 3.11 years. In March 2015, the Company granted 190,856 fully-vested ordinary shares to an executive of the Company and the Company recorded compensation expense in the amount of $0.9 million. Share-based compensation expense related to these fully-vested ordinary shares is included in general and administrative expenses on the consolidated statements of operations. Share-based compensation expense for the years ended December 31, 2017, 2016 and 2015 is classified in the consolidated statements of operations as follows: For the Year Ended December 31, 2017 2016 2015 (in thousands) Research and development expenses $ 7,670 $ 4,936 $ 2,268 General and administrative expenses 4,473 1,911 1,756 Total share-based compensation expense $ 12,143 $ 6,847 $ 4,024 Of the total share-based compensation expense recorded for the years ended December 31, 2017, 2016 and 2015, $2.9 million, $2.7 million and $1.6 million related to options granted to non-employees, respectively, all of which is included in research and development expenses on the consolidated statements of operations.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2017
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. In connection with the lease agreement, the Company is entitled to receive $11.5 million of tenant improvement allowances. The Company has received $3.6 million as of December 31, 2017, which is amortized over the period from the commencement of tenant improvement construction through to the end of the 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. In connection with the lease, the Company issued the lessor a letter of credit in the amount of $1.0 million, which is recorded as restricted cash on the consolidated balance sheets at December 31, 2017 Previously, the Company leased its corporate office space in Boston, Massachusetts under a non-cancellable operating sublease with SNBL, a related party. On September 22, 2015, the Company terminated its sublease with SNBL and exited the premises on October 2, 2015. As a result of the termination of the sublease, the Company recorded approximately $0.2 million of additional depreciation and $0.1 million of exit costs during the year ended December 31, 2015. In connection with the termination, the Company agreed to guarantee SNBL certain obligations of an unrelated third party who entered into a sublease agreement with SNBL effective October 2, 2015. The guarantee provides that in the event the sub-lessee does not meet its lease obligations to SNBL, the Company will make the required payments. The guarantee agreement is effective through August 2019, when the final lease payments are due, and coincides with the original expiration of the lease. The Company simultaneously entered into an indemnification agreement with the sub-lessee to indemnify the Company for any costs incurred under the guarantee made by the Company to SNBL. The maximum amount of the guarantee over the three year and six month sub-lease period is $0.6 million, exclusive of any indemnification from the sub-lessee. Future minimum lease payments under the Company’s non-cancelable operating leases as of December 31, 2017 For the Year Ended December 31, Amount (in thousands) 2018 4,666 2019 5,675 2020 5,846 2021 6,021 2022 6,201 Thereafter 26,163 54,572 The Company recorded rent expense of $5.6 million, $1.5 million and $0.5 million for the years ended December 31, 2017 Capital Lease In April 2015, the Company entered into a three year lease to acquire laboratory equipment, which has been accounted for as a capital lease. The capital asset was valued at $0.3 million and is included in property and equipment, net, along with accumulated amortization of $0.1 million as of December 31, 2017 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

Net Loss Per Ordinary Share	12 Months Ended
	Dec. 31, 2017
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, 2017 2016 2015 (in thousands except share and per share data) Numerator: Net loss attributable to ordinary shareholders $ (102,035 ) $ (55,401 ) $ (19,200 ) Denominator: Weighted-average ordinary shares outstanding 26,513,382 22,800,628 10,501,455 Net loss per share, basic and diluted $ (3.85 ) $ (2.43 ) $ (1.83 ) 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, 2017 2016 Options to purchase ordinary shares 3,767,130 3,577,766 Restricted share units 154,459 22,750 Series A preferred shares 3,901,348 3,901,348

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income Taxes	10. INCOME TAXES The components of loss before income taxes were as follows: Year Ended December 31, 2017 2016 2015 (in thousands) Singapore $ (76,885 ) $ (53,387 ) $ (16,534 ) Rest of world (24,442 ) (1,398 ) (2,622 ) Loss before income taxes $ (101,327 ) $ (54,785 ) $ (19,156 ) During the years ended December 31, 2017 U.S. income generated under research and management services arrangements between the Company’s U.S. and Singapore entities U.S. income generated between the Company’s U.S. and Singapore entities On October 1, 2017, the Company made changes to its corporate entity operating structure, including transferring intellectual property from the Japanese subsidiary to the Singapore parent company, as well as transferring intellectual property from the Singapore parent company to the U.S. and UK subsidiaries, primarily to align the Company’s intellectual property holding and management structure with its business functions. The transfer of assets occurred between wholly-owned legal entities within the Wave group that are all based in different tax jurisdictions. As the impact of the transfer was the result of an intra-entity transaction, any resulting gain or loss and immediate tax impact on the transfer is eliminated and not recognized in the consolidated financial statements under U.S. GAAP. The recipient entities will receive a tax benefit associated with the future amortization of the intellectual property received in accordance with the applicable tax laws. As discussed in Note 2, the Company will adopt ASU 2016-16 in the first quarter of 2018 and the Company estimates that there will be a cumulative-effect increase of approximately $0.4 million to the Company’s accumulated deficit. During the years ended December 31, 2017 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, 2017 and 2016. the Company established a wholly-owned subsidiary in the UK, however, during the year ended December 31, 2017 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, 2017, the Company has accounted for the impacts of the Tax Act to the extent a reasonable estimate could be made. The Company recognized a $0.8 million provisional charge related to the remeasurement of the Company’s deferred tax assets and liabilities, which 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 Company will continue to refine its estimates throughout the measurement period or until the accounting is complete as allowed under SAB 118. The components of the benefit (provision) for income taxes were as follows: Year Ended December 31, 2017 2016 2015 (in thousands) Current benefit (provision) for income taxes: Singapore taxes $ 199 $ — $ — Rest of world taxes (133 ) (1,180 ) (8 ) Total current benefit (provision) for income taxes $ 66 $ (1,180 ) $ (8 ) Deferred benefit (provision) for income taxes: Singapore taxes $ — $ — $ — Rest of world taxes (774 ) 564 (36 ) Total deferred benefit (provision) for income taxes $ (774 ) $ 564 $ (36 ) Total benefit (provision) for income taxes $ (708 ) $ (616 ) $ (44 ) A reconciliation of the Singapore statutory income tax rate to the Company’s effective income tax rate is as follows: Year Ended December 31, 2017 2016 2015 Singapore statutory income tax rate 17.0 % 17.0 % 17.0 % Federal and state tax credits 5.7 3.1 2.3 Permanent differences (2.6 ) (0.9 ) 5.5 Changes in reserves for uncertain tax positions (3.5 ) (3.6 ) (1.2 ) Foreign rate differential 2.8 (0.1 ) 1.2 Tax rate change (0.9 ) — — Other 0.5 (0.9 ) 0.2 Change in deferred tax asset valuation allowance (19.7 ) (15.7 ) (25.2 ) Effective income tax rate (0.7 )% (1.1 )% (0.2 )% The components of the Company’s deferred tax assets as of December 31, 2017 December 31, 2017 2016 (in thousands) Deferred tax assets: Net operating loss carryforwards $ 28,913 $ 16,046 Federal and state tax credits 4,522 449 Accrued expenses 1,903 242 Share-based compensation 1,921 1,024 Other 176 102 Total deferred tax assets 37,435 17,863 Valuation allowance (36,069 ) (15,999 ) Net deferred tax assets 1,366 1,864 Deferred tax liabilities: Depreciation (1,366 ) (1,090 ) Total deferred tax liabilities (1,366 ) (1,090 ) Net deferred tax assets (liabilities) $ — $ 774 A roll-forward of the valuation allowance for the years ended December 31, 2017 Year Ended December 31, 2017 2016 (in thousands) Balance at beginning of year $ 15,999 $ 7,466 Increase in valuation allowance 20,595 8,774 Reversal of valuation allowance (598 ) (282 ) Effect of foreign currency translation 73 41 Balance at end of year $ 36,069 $ 15,999 As of December 31, 2017 December 31, 2017 As of December 31, 2017 As of December 31, 2017 As of December 31, 2017, the Company has net operating loss carryforwards in the UK of $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, 2016, 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 Japan and Singapore. Accordingly, a full valuation allowance has been established against those deferred tax assets as of December 31, 2016. Additionally as of December 31, 2016, management has considered the Company’s expected utilization of U.S. Accordingly, a full valuation allowance has been established against those deferred tax assets as of . The valuation allowance increased by approximately $20.1 million in 2017, $8.5 million in 2016 and $4.8 million in 2015 primarily as a result of operating losses generated with no corresponding financial statement benefit. 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: 2017 2016 2015 (in thousands) Unrecognized tax benefit at the beginning of the year $ 2,343 $ 1,280 $ 1,025 Tax positions released related to prior years — (1,066 ) — Tax positions related to the current year 3,864 2,129 255 Unrecognized tax benefit at the end of the year $ 6,207 $ 2,343 $ 1,280 As of December 31, 2017 December 31, 2017 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, 2017 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 taxing authorities in the U.S., Japan, and Singapore. There are currently no pending income tax examinations. Tax years from 2012 to the present are still open to examination in the U.S., from 2008 to the present in Japan, and from 2012 to the present in Singapore. 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, 2017 Utilization of the net operating loss carryforwards and research and development tax credit carryforwards in the U.S. 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 2015, 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 research and development credit carryforwards to offset future tax liabilities. Should an ownership change have occurred after December 31, 2015 or occur in the future, the Company’s ability to utilize research and development tax credit carryforwards may be limited.

Employee Benefit Plans

Employee Benefit Plans	12 Months Ended
	Dec. 31, 2017
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 all U.S.-based employees. 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 $0.4 million in the year ended December 31, 2017. The Company did not make contributions to the 401(k) Plan during the years ended December 31, 2016 or 2015.

Related Parties

Related Parties	12 Months Ended
	Dec. 31, 2017
Related Party Transactions [Abstract]
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, December 31, 2017 • Pursuant to the terms of various service agreements with SNBL, the Company paid SNBL $0.5 million, $0.4 million and $0.1 million for the years ended December 31, 2017 • 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.

Geographic Data

Geographic Data	12 Months Ended
	Dec. 31, 2017
Text Block [Abstract]
Geographic Data	13. GEOGRAPHIC DATA The Company’s long-lived assets consist of property and equipment, net, and are located in the following geographical areas: December 31, 2017 December 31, 2016 (in thousands) Japan $ 14 $ 136 United States 27,320 8,471 Total long-lived assets $ 27,334 $ 8,607

Selected Quarterly Financial In

Selected Quarterly Financial Information (Unaudited)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Selected Quarterly Financial Information (Unaudited)	14. SELECTED QUARTERLY FINANCIAL INFORMATION (UNAUDITED) Selected quarterly results from operations for the years ended December 31, 2017 2017 Quarter Ended March 31 June 30 September 30 December 31 (in thousands, except for per share data) Revenues $ 676 $ 676 $ 676 $ 1,676 Operating expenses 20,590 25,770 27,668 32,256 Loss from operations (19,914 ) (25,094 ) (26,992 ) (30,580 ) Net loss (20,996 ) (24,693 ) (26,135 ) (30,211 ) Basic and diluted net loss per ordinary share $ (0.89 ) $ (0.92 ) $ (0.94 ) $ (1.09 ) 2016 Quarter Ended March 31 June 30 September 30 December 31 (in thousands, except for per share data) Revenues $ — $ 417 $ 392 $ 676 Operating expenses 7,952 12,055 17,625 19,180 Loss from operations (7,952 ) (11,638 ) (17,233 ) (18,504 ) Net loss (7,847 ) (11,565 ) (17,535 ) (18,454 ) Basic and diluted net loss per ordinary share $ (0.36 ) $ (0.51 ) $ (0.75 ) $ (0.79 )

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2017
Subsequent Events [Abstract]
Subsequent Events	15. SUBSEQUENT EVENTS Takeda Collaboration and License Agreement In February 2018, two of the Company’s subsidiaries entered into a global strategic collaboration (the “Takeda Collaboration”) that provides Takeda Pharmaceutical Company Limited (“Takeda”) with the option to co-develop and co-commercialize the Company’s CNS development programs in Huntington’s disease, amyotrophic lateral sclerosis and frontotemporal dementia, as well as a discovery stage program targeting ATXN3 for the treatment of spinocerebellar ataxia type 3. In addition, Takeda has the right to license multiple preclinical programs for CNS indications including Alzheimer’s disease and Parkinson’s disease. Subject to customary closing conditions, including the expiration or early termination of the applicable waiting period under the Hart-Scott-Rodino Antitrust Improvements Act of 1976 (the “HSR Act”), the Takeda Collaboration is expected to become effective during the first quarter of 2018. Simultaneously with the Company’s entry into the Takeda Collaboration Agreement, the Company entered into a share purchase agreement with Takeda pursuant to which the Company agreed to sell to Takeda 1,096,892 of its ordinary shares at a purchase price of $54.70 per share, for an aggregate purchase price of approximately $60.0 million (the “Takeda Equity Investment”). Subject to customary closing conditions, including the expiration or early termination of the applicable waiting period under the HSR Act, the Takeda Equity Investment is expected to close during the first quarter of 2018.

Significant Accounting Polici24

Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Cash Equivalents	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	Principles of Consolidation The Company’s consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries. All significant intercompany balances and transactions have been eliminated in consolidation.
Use of Estimates	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 Series A preferred shares on conversion of the related party notes payable, 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 period over which revenue is recognized under the Pfizer Collaboration Agreement (as defined in Note 5)
Segment Data	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 synthetic chemistry platform to develop and commercialize a broad pipeline of nucleic acid-based therapeutics.
Foreign Currency Translation	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. Prior to 2017, Wave Japan had intercompany loans payable to Wave that were not expected to be settled in the foreseeable future which were therefore translated at the historical rate for the date of each capital transaction. In 2017, Wave Japan repaid the intercompany loans which resulted in a foreign exchange loss which is included in the consolidated statements of operations within other income (expense), net. Net unrealized gains and losses from foreign currency translation are reflected as accumulated other comprehensive (loss) income within Series A preferred shares and shareholders’ equity and consolidated statements of comprehensive loss. Gains and losses on foreign currency transactions are included in the consolidated statements of operations within other income (expenses), net.
Fair Value of Financial Instruments	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 and cash equivalents are Level 1 assets which are comprised of funds held in readily available checking and money market accounts.
Concentration of Credit Risk	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 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, 2017 and 2016, the Company had $3.6 million of restricted cash, of which $2.6 million related to the Lexington facility and the remaining $1.0 million related to the Cambridge facility.
Property and Equipment	Property and Equipment Property and equipment, which consists of furniture and equipment and leasehold improvements are stated at cost less accumulated depreciation and amortization. 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 life of lease or useful life Depreciation and amortization 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.
Impairment of Long-Lived Assets	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. Through December 31, 2017, the Company has not recognized any impairment charges.
Revenue Recognition	Revenue Recognition As of December 31, 2017, the Company’s only significant source of revenue is derived from the Pfizer Collaboration Agreement (as defined in Note 5), pursuant to which the Company and Pfizer (as defined in Note 5) have agreed to collaborate on the discovery, development and commercialization of stereopure oligonucleotide therapeutics for the Pfizer Programs (as defined in Note 5), each directed at a genetically-defined hepatic target selected by Pfizer. The Company entered into the Pfizer Collaboration Agreement in May 2016. The Company presents revenue from the Pfizer Collaboration Agreement under Financial Accounting Standards Board (“FASB”), Accounting Standards Codification (“ASC”) Topic 808, Collaborative Arrangements (“ASC 808”). In addition, the Company recognizes revenue in accordance with ASC Topic 605, Revenue Recognition (“ASC 605”). Accordingly, revenue is recognized for each unit of accounting when all of the following criteria are met: • persuasive evidence of an arrangement exists; • delivery has occurred or services have been rendered; • the seller’s price to the buyer is fixed or determinable; and • collectability is reasonably assured. Amounts received prior to satisfying the revenue recognition criteria 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. Pursuant to the accounting guidance in ASC 605‑25, the Company evaluates multiple‑element arrangements to determine (1) the deliverables included in the arrangement and (2) whether the individual deliverables represent separate units of accounting or whether they must be accounted for as a combined unit of accounting. This evaluation involves subjective determinations and requires the Company to make judgments about the individual deliverables and whether such deliverables are separable from the other aspects of the contractual relationship. Deliverables are considered separate units of accounting provided that the delivered item has value to the customer on a standalone basis and, if the arrangement includes a general right of return relative to the delivered item, delivery or performance of the undelivered item is considered probable and substantially in the Company’s control. In assessing whether an item has standalone value, the Company considers factors such as the research, development, manufacturing and commercialization capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. In addition, the Company considers whether the collaboration partner can use a deliverable for its intended purpose without the receipt of the remaining deliverable, whether the value of the deliverable is dependent on the undelivered item and whether there are other vendors that can provide the undelivered items. Under the Pfizer Collaboration Agreement, the Company and Pfizer agreed to collaborate on the discovery, development and commercialization of up to five Pfizer Programs, two of the five targets were declared upon initiation of the agreement in May 2016. The Pfizer Collaboration Agreement provides Pfizer with certain options to nominate up to three remaining programs and the Company is required to consider whether such options are substantive. Options are considered substantive if, at the inception of the arrangement, the Company is at risk as to whether the collaboration partner will choose to exercise the option. Factors that the Company considers in evaluating whether an option is substantive include whether the optional elements are essential to the functionality of other programs nominated, whether economic factors compel Pfizer to purchase the optional elements, the cost to exercise the option, the overall objective of the arrangement and, the benefit Pfizer might obtain from the arrangement without exercising the option. In August 2016, Pfizer nominated the third hepatic target under the Collaboration and pursuant to the terms of the Pfizer Collaboration Agreement, Pfizer had the option to nominate two additional targets by November 5, 2017. On November 5, 2017, the Company amended its Pfizer Collaboration Agreement to extend the target nomination period from November 5, 2017 to May 5, 2018. This amendment provides Pfizer with an additional six months to nominate the two remaining hepatic targets under the Pfizer Collaboration Agreement. When an option is considered substantive and there is no significant incremental discount, the option is not considered a deliverable in the arrangement and no consideration is allocated to it. Conversely, when an option is not considered substantive or it is considered substantive but is priced at an incremental discount, it is analyzed to determine if it should be combined with other deliverables in the arrangement. Options that are substantive and priced at a significant and incremental discount are further assessed to determine whether a portion of the upfront payment should be allocated to the option and other deliverables in the arrangement. At the inception of an arrangement that includes milestone payments, the Company evaluates whether each milestone is substantive and at risk to both parties on the basis of the contingent nature of the milestone. This evaluation includes an assessment of whether: (1) the consideration is commensurate with either the Company’s performance to achieve the milestone or the enhancement of the value of the delivered item(s) as a result of a specific outcome resulting from its performance to achieve the milestone, (2) the Aside from the program nomination payments, which relate to the options described above, the remaining milestone payments required under the Pfizer Collaboration Agreement are contingent upon the Company’s performance under the Pfizer Collaboration Agreement, including in certain instances, regulatory approval. The Company views these milestones as substantive and has excluded the amounts as allocable consideration at the outset of the arrangement. All commercial milestones will be accounted for in the same manner as royalties and recorded as revenue upon achievement of the milestone, assuming all other revenue recognition criteria are met. The Company recognizes arrangement consideration allocated to each unit of accounting when all of the revenue recognition criteria in ASC 605 are satisfied for that particular unit of accounting. In the event that a deliverable does not represent a separate unit of accounting, the Company recognizes revenue from the combined unit of accounting over the Company’s contractual or estimated performance period for the undelivered elements, which is typically the term of the Company’s research and development obligations. If there is no discernible pattern of performance or objectively measurable performance measures do not exist, then the Company recognizes revenue under the arrangement on a straight‑line basis over the period the Company is expected to complete its performance obligations. Conversely, if the pattern of performance in which the service is provided to the customer can be determined and objectively measurable performance measures exist, then the Company recognizes revenue under the arrangement using the proportional performance method. Revenue recognized is limited to the lesser of the cumulative amount of payments received or the cumulative amount of revenue earned, as determined using the straight‑line method or proportional performance method, as applicable, as of the period ending date. The Company has concluded that the deliverables under the Pfizer Collaboration Agreement relate primarily to the research and development required by the Company for each of the programs nominated by Pfizer. The remaining deliverables, including sample supplies provided by each party to fulfill its obligation as a licensee, participation on a joint steering committee to oversee the research and development activities, and regulatory responsibilities related to filings and obtaining approvals related to the products that may result from each program do not represent separate units of accounting based on their dependence on the research and development efforts. Because there is no discernible pattern of performance given the nature of the research and development efforts, the Company recognizes the allocated revenue for each deliverable under the Pfizer Collaboration Agreement on a straight ‑ Product Revenue The Company has had no product revenue to date.
Net Loss per Share	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	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.
Share-Based Compensation	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 issuances of 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 in the same manner in which the award recipient’s payroll costs are classified or in which the award recipients’ service payments are classified. Prior to the Company’s initial public offering (“IPO”) in November 2015, the fair value of the ordinary shares underlying its share-based awards was estimated on each grant date by the board of directors. The board of directors determined the estimated per share fair value of the Company’s ordinary shares at various dates considering contemporaneous and retrospective valuations performed in accordance with the guidance outlined in the American Institute of Certified Public Accountants Practice Aid, Valuation of Privately-Held Company Equity Securities Issued as Compensation The fair value of each share option grant was determined using the methods and assumptions discussed below. Each of these inputs is subjective and generally requires significant judgment and estimation by management. • Fair Value of Ordinary Shares . As discussed above, prior to the Company’s IPO, the fair value of the Company’s ordinary shares underlying the Company’s share options was historically determined by the board of directors. Because prior to the Company’s IPO, there was no public market for the Company’s ordinary shares, the board of directors determined the fair value of the Company’s ordinary shares at the time of grant of the option by considering a number of objective and subjective factors, including valuations of comparable companies, sales of its shares to unrelated third parties, operating and financial performance and general and industry specific economic outlook. Following the completion of the Company’s IPO, 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	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 taxing 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. 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 the related ongoing results of the comparable companies.
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements In May 2014, the FASB issued Accounting Standards Update (“ASU”) No. 2014-09, Revenue from Contracts with Customers (Topic 606) (“ASU 2014-09”), which supersedes the revenue recognition requirements in ASC 605-25, Multiple-Element Arrangements Under ASC 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which 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 steps: (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. ASC 606 also impacts certain other areas, such as the accounting for costs to obtain or fulfill a contract. The standard also requires disclosure of the nature, amount, timing, and uncertainty of revenue and cash flows arising from contracts with customers. The Company is assessing but has not yet completed its assessment of the impact of the adoption of this standard on its consolidated financial statements. Therefore, the Company does not know and cannot reasonably estimate the impact that adoption of ASC 606 is expected to have on the consolidated financial statements. Currently, the Company anticipates a potential impact on the revenue recognition method used to recognize revenue for the identified performance obligations under the Pfizer Collaboration Agreement as well as the recognition of milestone revenue prior to achievement. The expected impact is further described below. Estimated impacts from the adoption of this standard could differ upon the final adoption and implementation of the standard. With respect to the Pfizer Collaboration Agreement, the Company currently expects the five performance obligations identified under the provisions of ASC 606 will be consistent with the five units of accounting identified under the provisions of ASC 605. However, as previously described, it currently expects that the timing and pattern of revenue recognition under step (v) above will differ from the pattern of revenue recognition under ASC 605. Under ASC 606, the revenues will be recognized over time. As of December 31, 2017, the Company had recognized $5.2 million of revenue under the Pfizer Collaboration Agreement. Deferred revenue related to the Pfizer Collaboration Agreement amounted to $8.3 million as of December 31, 2017, of which $2.7 million is included in current liabilities. The Company expects a change in the timing and pattern of revenue recognition upon adoption of ASC 606 to impact the Company’s revenue, deferred revenue and net loss. The Company expects the accounting for contingent milestone payments under its collaboration agreements to change under ASC 606. ASC 606 does not contain guidance specific to milestone payments, thereby requiring contingent milestone payments to be considered in accordance with the overall model of ASC 606. Revenue from contingent milestone payments may be recognized earlier under ASC 606 than under ASC 605, based on an assessment of the probability of achievement of the milestone event and the likelihood of a significant reversal of such milestone revenue at each reporting date. This assessment may result in the recognition of revenue related to a contingent milestone payment before the milestone event has been achieved. ASC 606 requires more robust disclosures than required by previous guidance, including disclosures related to disaggregation of revenue into appropriate categories, performance obligations, the judgments made in revenue recognition determinations, adjustments to revenue which relate to activities from previous quarters or years, any significant reversals of revenue, and costs to obtain or fulfill contracts. In connection with the adoption of these standards, the Company is implementing several new internal controls, including controls to monitor the probability of achievement of contingent milestone payments and the timing and pattern of performance of the performance obligation. In February 2016, the FASB issued Accounting Standards Update No. 2016-02, Leases , 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”), which requires an entity 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. These amendments should be applied on a modified retrospective basis through a cumulative-effect adjustment directly to retained earnings at the beginning of the period adopted. The Company will adopt ASU 2016-16 in the first quarter of 2018 and the Company estimates that there will be a cumulative-effect increase of approximately $0.4 million to the Company’s accumulated deficit. In November 2016, the FASB issued Accounting Standards Update No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash (“ASU 2016-18”). The ASU requires an entity to explain the changes in the total of cash, cash equivalents, restricted cash, and restricted cash equivalents on the statement of cash flows and to provide a reconciliation of the totals in that statement to the related captions in the balance sheet when the cash, cash equivalents, restricted cash, and restricted cash equivalents are presented in more than one line item on the balance sheet. This ASU is effective for annual and interim periods beginning after December 15, 2017, and is required to be adopted using a retrospective approach, with early adoption permitted. The Company is currently evaluating the potential impact that the adoption of ASU 2016-18 may have on its consolidated financial statements. Other accounting standards that have been issued or proposed by the FASB or other standards-setting bodies that do not require adoption until a future date are not expected to have a material impact on the Company’s consolidated financial statements upon adoption. Recently Adopted Accounting Pronouncements In November 2015, the FASB issued Accounting Standards Update No. 2015-17, Balance Sheet Classification of Deferred Taxes (“ASU 2015-17”),

Significant Accounting Polici25

Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
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 life of lease or useful life

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2017
Property Plant And Equipment [Abstract]
Schedule of Property and Equipment, Net	Property and equipment, net, consists of the following December 31, 2017 2016 (in thousands) Furniture and equipment $ 13,626 $ 7,231 Software 108 43 Leasehold improvements 16,029 1,964 Fixed assets in progress 1,988 1,863 Total 31,751 11,101 Less accumulated depreciation and amortization (4,417 ) (2,494 ) Property and equipment, net $ 27,334 $ 8,607

Accrued Expenses and Other Cu27

Accrued Expenses and Other Current Liabilities (Tables)	12 Months Ended
	Dec. 31, 2017
Payables And Accruals [Abstract]
Summary of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consist of the following: December 31, 2017 2016 (in thousands) Accrued compensation $ 5,428 $ 2,480 Accrued professional fees 3,281 417 Accrued vacation 33 589 Other current liabilities 156 948 Total accrued expenses and other current liabilities $ 8,898 $ 4,434

Share-Based Compensation (Table

Share-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 3,577,766 $ 10.58 Granted 522,750 26.33 Exercised (137,493 ) 6.74 Forfeited or cancelled (195,893 ) 14.71 Outstanding as of December 31, 2017 3,767,130 $ 12.69 7.81 $ 84,675 Options exercisable as of December 31, 2017 2,257,455 $ 7.64 7.43 $ 62,060 Options unvested as of December 31, 2017 1,509,675 $ 20.23 8.37 $ 22,614 (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 years ended December 31, 2017 and 2016 is summarized as follows: RSUs Average Grant Date Fair Value (in dollars per share) Outstanding as of January 1, 2017 22,750 21.69 Granted 170,859 29.05 Vested (22,750 ) 21.69 Forfeited (16,400 ) 29.05 RSUs Outstanding at December 31, 2017 154,459 $ 29.05
Summary of Share-based Compensation Expense Classified in Consolidated Statements of Operations	Share-based compensation expense for the years ended December 31, 2017, 2016 and 2015 is classified in the consolidated statements of operations as follows: For the Year Ended December 31, 2017 2016 2015 (in thousands) Research and development expenses $ 7,670 $ 4,936 $ 2,268 General and administrative expenses 4,473 1,911 1,756 Total share-based compensation expense $ 12,143 $ 6,847 $ 4,024
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, 2017 2016 2015 Risk-free interest rate 1.49% – 2.23% 1.15% – 2.18% 1.56% - 1.89% Expected term (in years) 3.00 – 6.25 3.00 – 6.25 5.52 - 6.12 Expected volatility 68.95% – 72.24% 60.89% – 68.76% 62.14% - 71.02% Expected dividend yield 0% 0% 0%
Non-Employees 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 non-employees during the period were as follows: Year Ended December 31, 2015 Risk-free interest rate 2.06% - 2.35% Expected term (in years) 9.19 - 10.00 Expected volatility 62.65% - 69.80% Expected dividend yield 0%

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 For the Year Ended December 31, Amount (in thousands) 2018 4,666 2019 5,675 2020 5,846 2021 6,021 2022 6,201 Thereafter 26,163 54,572

Net Loss Per Ordinary Share (Ta

Net Loss Per Ordinary Share (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 2016 2015 (in thousands except share and per share data) Numerator: Net loss attributable to ordinary shareholders $ (102,035 ) $ (55,401 ) $ (19,200 ) Denominator: Weighted-average ordinary shares outstanding 26,513,382 22,800,628 10,501,455 Net loss per share, basic and diluted $ (3.85 ) $ (2.43 ) $ (1.83 )
Anti-Dilutive Shares Excluded from Calculation of Diluted Net Loss Per Ordinary Share	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, 2017 2016 Options to purchase ordinary shares 3,767,130 3,577,766 Restricted share units 154,459 22,750 Series A preferred shares 3,901,348 3,901,348

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2017
Components of Loss Before Income Taxes	The components of loss before income taxes were as follows: Year Ended December 31, 2017 2016 2015 (in thousands) Singapore $ (76,885 ) $ (53,387 ) $ (16,534 ) Rest of world (24,442 ) (1,398 ) (2,622 ) Loss before income taxes $ (101,327 ) $ (54,785 ) $ (19,156 )
Deferred Components of Benefit (Provision) for Income Taxes	The components of the benefit (provision) for income taxes were as follows: Year Ended December 31, 2017 2016 2015 (in thousands) Current benefit (provision) for income taxes: Singapore taxes $ 199 $ — $ — Rest of world taxes (133 ) (1,180 ) (8 ) Total current benefit (provision) for income taxes $ 66 $ (1,180 ) $ (8 ) Deferred benefit (provision) for income taxes: Singapore taxes $ — $ — $ — Rest of world taxes (774 ) 564 (36 ) Total deferred benefit (provision) for income taxes $ (774 ) $ 564 $ (36 ) Total benefit (provision) for income taxes $ (708 ) $ (616 ) $ (44 )
Components of Deferred Tax Assets	The components of the Company’s deferred tax assets as of December 31, 2017 December 31, 2017 2016 (in thousands) Deferred tax assets: Net operating loss carryforwards $ 28,913 $ 16,046 Federal and state tax credits 4,522 449 Accrued expenses 1,903 242 Share-based compensation 1,921 1,024 Other 176 102 Total deferred tax assets 37,435 17,863 Valuation allowance (36,069 ) (15,999 ) Net deferred tax assets 1,366 1,864 Deferred tax liabilities: Depreciation (1,366 ) (1,090 ) Total deferred tax liabilities (1,366 ) (1,090 ) Net deferred tax assets (liabilities) $ — $ 774
Summary of Valuation Allowance	A roll-forward of the valuation allowance for the years ended December 31, 2017 Year Ended December 31, 2017 2016 (in thousands) Balance at beginning of year $ 15,999 $ 7,466 Increase in valuation allowance 20,595 8,774 Reversal of valuation allowance (598 ) (282 ) Effect of foreign currency translation 73 41 Balance at end of year $ 36,069 $ 15,999
Summary of Activity in Unrecognized Tax Benefits	A summary of activity in the Company’s unrecognized tax benefits is as follows: 2017 2016 2015 (in thousands) Unrecognized tax benefit at the beginning of the year $ 2,343 $ 1,280 $ 1,025 Tax positions released related to prior years — (1,066 ) — Tax positions related to the current year 3,864 2,129 255 Unrecognized tax benefit at the end of the year $ 6,207 $ 2,343 $ 1,280
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, 2017 2016 2015 Singapore statutory income tax rate 17.0 % 17.0 % 17.0 % Federal and state tax credits 5.7 3.1 2.3 Permanent differences (2.6 ) (0.9 ) 5.5 Changes in reserves for uncertain tax positions (3.5 ) (3.6 ) (1.2 ) Foreign rate differential 2.8 (0.1 ) 1.2 Tax rate change (0.9 ) — — Other 0.5 (0.9 ) 0.2 Change in deferred tax asset valuation allowance (19.7 ) (15.7 ) (25.2 ) Effective income tax rate (0.7 )% (1.1 )% (0.2 )%

Geographic Data (Tables)

Geographic Data (Tables)	12 Months Ended
	Dec. 31, 2017
Text Block [Abstract]
Summary of Long-Lived Assets by Geographical Areas	The Company’s long-lived assets consist of property and equipment, net, and are located in the following geographical areas: December 31, 2017 December 31, 2016 (in thousands) Japan $ 14 $ 136 United States 27,320 8,471 Total long-lived assets $ 27,334 $ 8,607

Selected Quarterly Financial 33

Selected Quarterly Financial Information (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Schedule of Selected Quarterly Results from Operations	Selected quarterly results from operations for the years ended December 31, 2017 2017 Quarter Ended March 31 June 30 September 30 December 31 (in thousands, except for per share data) Revenues $ 676 $ 676 $ 676 $ 1,676 Operating expenses 20,590 25,770 27,668 32,256 Loss from operations (19,914 ) (25,094 ) (26,992 ) (30,580 ) Net loss (20,996 ) (24,693 ) (26,135 ) (30,211 ) Basic and diluted net loss per ordinary share $ (0.89 ) $ (0.92 ) $ (0.94 ) $ (1.09 ) 2016 Quarter Ended March 31 June 30 September 30 December 31 (in thousands, except for per share data) Revenues $ — $ 417 $ 392 $ 676 Operating expenses 7,952 12,055 17,625 19,180 Loss from operations (7,952 ) (11,638 ) (17,233 ) (18,504 ) Net loss (7,847 ) (11,565 ) (17,535 ) (18,454 ) Basic and diluted net loss per ordinary share $ (0.36 ) $ (0.51 ) $ (0.75 ) $ (0.79 )

The Company - Additional Inform

The Company - Additional Information (Detail) - USD ($) $ in Thousands	May 05, 2016	Apr. 30, 2017	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate net proceeds from private placement, public offering and collaborations				$ 323,200
Net proceeds from private placements				89,300
Proceeds from initial public offering, net of offering costs and underwriter commissions						$ 101,444
Issuance of ordinary shares, net of offering costs				93,509	$ 30,000	11,631
Proceeds from issuance of ordinary shares, net of offering costs				93,509	$ 30,000	$ 11,631
Research, License and Option Agreement [Member] | Pfizer Inc. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Proceeds from collaborations				40,000
Upfront amount related to research and development services	$ 10,000			10,000
Issuance of ordinary shares, net of offering costs	$ 30,000			30,000
Underwriters Over-Allotment Option [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Proceeds from initial public offering, net of offering costs and underwriter commissions			$ 100,400	100,400
Gross proceeds from sale of common stock				111,900
Follow-On Underwritten Public Offering [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Gross proceeds from sale of ordinary shares		$ 100,000		100,000
Proceeds from issuance of ordinary shares, net of offering costs		$ 93,500		$ 93,500

Significant Accounting Polici35

Significant Accounting Policies - Additional Information (Detail)	May 05, 2016Program	Dec. 31, 2017USD ($)Segment	Mar. 31, 2018USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)
Significant Accounting Policies [Line Items]
Number of operating segment | Segment		1
Cash and cash equivalents		$ 142,503,000		$ 150,293,000	$ 161,220,000	$ 1,048,000
Restricted cash		3,610,000		3,601,000
Impairment charges		$ 0
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.
Current portion of deferred revenue		$ 2,705,000		$ 2,705,000
Accounting Standards Update No. 2016-16 [Member] | Scenario Forecast [Member]
Significant Accounting Policies [Line Items]
Cumulative-effect adjustment to retained earnings			$ 400,000
Minimum [Member]
Significant Accounting Policies [Line Items]
Income tax examination, likelihood of settlement, percentage		50.00%
Pfizer Inc. [Member]
Significant Accounting Policies [Line Items]
Number of research programs, counterparty nomination | Program	5
Revenue recognized		$ 5,200,000
Deferred revenue		8,300,000
Current portion of deferred revenue		2,700,000
Lexington, MA [Member]
Significant Accounting Policies [Line Items]
Restricted cash		2,600,000
Cambridge MA [Member]
Significant Accounting Policies [Line Items]
Restricted cash		$ 1,000,000

Significant Accounting Polici36

Significant Accounting Policies - Schedule of Estimated Useful Lives of Assets (Detail)	12 Months Ended
	Dec. 31, 2017
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 life of lease or useful life

Property and Equipment, Net - S

Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property Plant And Equipment [Line Items]
Property and equipment gross	$ 31,751	$ 11,101
Less accumulated depreciation and amortization	(4,417)	(2,494)
Property and equipment, net	27,334	8,607
Furniture and Equipment [Member]
Property Plant And Equipment [Line Items]
Property and equipment gross	13,626	7,231
Software [Member]
Property Plant And Equipment [Line Items]
Property and equipment gross	108	43
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Property and equipment gross	16,029	1,964
Fixed Assets in Progress [Member]
Property Plant And Equipment [Line Items]
Property and equipment gross	$ 1,988	$ 1,863

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property Plant And Equipment [Abstract]
Depreciation and amortization expense	$ 2,155	$ 784	$ 594

Accrued Expenses and Other Cu39

Accrued Expenses and Other Current Liabilities - Summary of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Accrued compensation	$ 5,428	$ 2,480
Accrued professional fees	3,281	417
Accrued vacation	33	589
Other current liabilities	156	948
Total accrued expenses and other current liabilities	$ 8,898	$ 4,434

Pfizer Collaboration and Shar40

Pfizer Collaboration and Share Purchase Agreement - Additional Information (Detail)	Aug. 31, 2016USD ($)	May 05, 2016USD ($)Program$ / sharesshares	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Equity investment aggregate purchase price			$ 93,509,000	$ 30,000,000	$ 11,631,000
Current portion of deferred revenue			$ 2,705,000	$ 2,705,000
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 the 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 to have exclusive rights to undertake the clinical development of the resulting clinical candidates into products and the potential commercialization of any such products thereafter.
Deferred revenue			$ 8,300,000
Collaboration agreement, additional period after research program		2 years
Revenue recognized			3,700,000
Current portion of deferred revenue			2,700,000
Research, License and Option Agreement [Member] | Pfizer Inc. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront amount related to research and development services		$ 10,000,000	10,000,000
Collaboration agreement refundable		$ 0
Shares issued under equity agreement | shares		1,875,000
Equity investment aggregate purchase price		$ 30,000,000	$ 30,000,000
Purchase price per share | $ / shares		$ 16
Collaborative agreement research term		4 years
Milestone payment received	$ 2,500,000
Upfront payment allocated	3,000,000	$ 7,000,000
Deferred revenue	3,500,000
Collaboration agreement termination period		90 days
Research, License and Option Agreement [Member] | Pfizer Inc. [Member] | Option at Inception [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	1,000,000
Research, License and Option Agreement [Member] | Pfizer Inc. [Member] | Option at Exercising [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	$ 2,500,000

Share Capital - Ordinary Shares

Share Capital - Ordinary Shares - Additional Information (Detail)	May 05, 2016USD ($)$ / sharesshares	Apr. 30, 2017USD ($)shares	Dec. 31, 2015USD ($)$ / sharesshares	Nov. 30, 2015$ / sharesshares	Mar. 31, 2015shares	Jan. 31, 2015USD ($)$ / sharesshares	Feb. 28, 2014USD ($)$ / sharesshares	Dec. 31, 2017USD ($)Vote / shares$ / sharesshares	Dec. 31, 2016USD ($)$ / sharesshares	Dec. 31, 2015USD ($)$ / sharesshares	Nov. 16, 2015shares	Sep. 30, 2015USD ($)	Aug. 14, 2015shares
Class of Stock [Line Items]
Shares issued								27,829,079	23,502,169
Proceeds from issuance of shares | $								$ 323,200,000
Notes payable, related parties | $							$ 9,600,000
Notes payable convert to shares							1,515,596
Proceeds from sale of common stock | $										$ 101,444,000
Equity investment aggregate purchase price | $								93,509,000	$ 30,000,000	11,631,000
Net proceeds from sale of ordinary shares | $								$ 93,509,000	$ 30,000,000	11,631,000
Common stock, par value | $ / shares								$ 0	$ 0
Ordinary share holder, voting right								One vote for each ordinary share
Voting rights, votes per share | Vote / shares								1
Preferred stock, aggregate liquidation preference | $								$ 10,000				$ 10,000
Singapore [Member]
Class of Stock [Line Items]
Common stock, par value | $ / shares
Common stock, authorized								0
Research, License and Option Agreement [Member] | Pfizer Inc. [Member]
Class of Stock [Line Items]
Shares issued under equity agreement	1,875,000
Purchase price per share | $ / shares	$ 16
Equity investment aggregate purchase price | $	$ 30,000,000							$ 30,000,000
IPO [Member]
Class of Stock [Line Items]
Number of shares issued and sold				6,375,000
Shares issued, price per share | $ / shares				$ 16
Convertible preferred stock converted into common stock											5,334,892
Equity investment aggregate purchase price | $										$ 100,366,000
Underwriters Over-Allotment Option [Member]
Class of Stock [Line Items]
Number of shares issued and sold			618,126
Shares issued, price per share | $ / shares			$ 16							$ 16
Proceeds from sale of common stock | $			$ 100,400,000					100,400,000
Follow-On Underwritten Public Offering [Member]
Class of Stock [Line Items]
Shares issued under equity agreement		4,166,667
Gross proceeds from sale of ordinary shares | $		$ 100,000,000						100,000,000
Net proceeds from sale of ordinary shares | $		$ 93,500,000						$ 93,500,000
Executive Officer [Member]
Class of Stock [Line Items]
Options to purchase common stock granted					190,856
Ordinary Shares [Member]
Class of Stock [Line Items]
Notes payable convert to shares							1,515,596
Shares issued under equity agreement								4,166,667	1,875,000	4,769,077
Equity investment aggregate purchase price | $								$ 93,509,000	$ 30,000,000	$ 11,631,000
Ordinary Shares [Member] | IPO [Member]
Class of Stock [Line Items]
Shares issued under equity agreement										6,993,126
Equity investment aggregate purchase price | $										$ 100,366,000
Series A Preferred Shares [Member]
Class of Stock [Line Items]
Notes payable convert to shares							2,365,139
Series B Preferred Shares [Member]
Class of Stock [Line Items]
Preferred stock, shares issued													5,334,892
Third-Party Investor [Member]
Class of Stock [Line Items]
Shares issued						4,769,077	2,263,291
Shares issued, Per Share | $ / shares						$ 2.47	$ 2.47
Proceeds from issuance of shares | $						$ 11,600,000	$ 5,600,000

Share Capital - Series A Prefer

Share Capital - Series A Preferred Shares - Additional Information (Detail) - USD ($)	Aug. 14, 2015	Feb. 28, 2014	Dec. 31, 2017	Sep. 30, 2015
Class of Stock [Line Items]
Notes payable, related parties		$ 9,600,000
Notes payable convert to shares		1,515,596
Minimum number of shares to be held to receive right to purchase new securities	1,212,477
Percentage of voting Right	50.00%
Preferred stock, aggregate liquidation preference			$ 10,000	$ 10,000
Series A Preferred Shares [Member]
Class of Stock [Line Items]
Notes payable convert to shares		2,365,139
Share issued for conversion	3,901,348
Preferred stock liquidation preference, per share				$ 0.002
Aggregate preferred stock liquidation preference, per share				$ 10
Series A Preferred Shares [Member] | Singapore [Member]
Class of Stock [Line Items]
Preferred stock, par value
Preferred stock, shares authorized			0
Series B Preferred Shares [Member]
Class of Stock [Line Items]
Preferred stock, voting rights	56.00%
Preferred stock liquidation preference, per share			$ 12.37

Share Capital - Series B Prefer

Share Capital - Series B Preferred Shares - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Aug. 14, 2015	Dec. 31, 2017	Dec. 31, 2015	Nov. 16, 2015
Class of Stock [Line Items]
Proceeds from issuance of preferred shares			$ 62,532
IPO [Member]
Class of Stock [Line Items]
Convertible preferred stock converted into common stock				5,334,892
Sale of common stock, Date		Nov. 16, 2015
Series B Preferred Shares [Member]
Class of Stock [Line Items]
Preferred stock, shares issued	5,334,892
Preferred shares issued, Per Share	$ 12.37
Proceeds from issuance of preferred shares	$ 62,500
Preferred stock liquidation preference, per share		$ 12.37

Share-Based Compensation - Addi

Share-Based Compensation - Additional Information (Detail) - USD ($)	1 Months Ended	12 Months Ended
	Mar. 31, 2015	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		$ 12,143,000	$ 6,847,000	$ 4,024,000
Research and Development Expenses [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation expense		$ 7,670,000	$ 4,936,000	$ 2,268,000
Executive Officer [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options to purchase common stock granted	190,856
Restricted Share Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period		4 years
Number of units, granted		170,859		0
Unrecognized stock-based compensation weighted average recognition period		3 years 1 month 9 days
Unrecognized stock-based compensation expense		$ 3,500,000
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)		6,064,544		5,064,544
Ordinary shares available for future grant		1,716,110
Options to purchase common stock granted		522,750
Unrecognized stock-based compensation weighted average recognition period		2 years 7 months 6 days
Weighted average grant date fair value per granted option		$ 16.58	$ 30.23
Aggregate fair value of options vested		$ 11,500,000
Share-based compensation expense	$ 900,000
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
Unrecognized stock-based compensation expense		$ 1,000,000
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		2,900	$ 2,700	$ 1,600
2014 Plan [Member] | Employee Stock Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation expense		$ 16,900,000
2014 Plan [Member] | Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period		3 years
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 - Summ

Share-Based Compensation - Summary of Share Option Activity (Detail) - 2014 Plan [Member] $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017USD ($)$ / sharesshares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding, Number of Shares, Beginning Balance | shares	3,577,766
Granted, Number of Shares | shares	522,750
Exercised, Number of Shares | shares	(137,493)
Forfeited or cancelled, Number of Shares | shares	(195,893)
Outstanding, Number of Shares, Ending Balance | shares	3,767,130
Options exercisable, Number of Shares | shares	2,257,455
Options unvested, Number of Shares | shares	1,509,675
Outstanding, Weighted Average Exercise Price, Beginning Balance | $ / shares	$ 10.58
Granted, Weighted Average Exercise Price | $ / shares	26.33
Exercised, Weighted Average Exercise Price | $ / shares	6.74
Forfeited or cancelled, Weighted Average Exercise Price | $ / shares	14.71
Outstanding, Weighted Average Exercise Price, Ending Balance | $ / shares	12.69
Options exercisable, Weighted Average Exercise Price | $ / shares	7.64
Options unvested, Weighted Average Exercise Price | $ / shares	$ 20.23
Outstanding, Weighted Average Remaining Contractual Term	7 years 9 months 21 days
Options exercisable, Weighted Average Remaining Contractual Term	7 years 5 months 4 days
Options unvested, Weighted Average Remaining Contractual Term	8 years 4 months 13 days
Outstanding, Aggregate Intrinsic Value | $	$ 84,675
Options exercisable, Aggregate Intrinsic Value | $	62,060
Options unvested, Aggregate Intrinsic Value | $	$ 22,614

Share-Based Compensation - Su46

Share-Based Compensation - Summary of Fair Value of Share Options Granted to Employees (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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	1.49%	1.15%	1.56%
Risk-free interest rate, maximum	2.23%	2.18%	1.89%
Expected volatility, minimum	68.95%	60.89%	62.14%
Expected volatility, maximum	72.24%	68.76%	71.02%
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	5 years 6 months 7 days
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 1 month 13 days

Share-Based Compensation - Su47

Share-Based Compensation - Summary of Fair Value of Share Options Granted to Non-Employees (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected dividend yield	0.00%
Non-Employees Stock Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate, minimum		2.06%
Risk-free interest rate, maximum		2.35%
Expected volatility, minimum		62.65%
Expected volatility, maximum		69.80%
Expected dividend yield		0.00%
Non-Employees Stock Option [Member] | Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)		9 years 2 months 9 days
Non-Employees Stock Option [Member] | Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)		10 years

Share-Based Compensation - Su48

Share-Based Compensation - Summary of RSU Activity (Detail) - Restricted Share Units [Member] - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding, Number of Shares, Beginning Balance	22,750
Granted, Number of Shares	170,859	0
Vested, Number of Shares	(22,750)
Forfeited, Number of Shares	(16,400)
Outstanding, Number of Shares, Ending Balance	154,459
Outstanding, Average Grant Date Fair Value, Beginning Balance	$ 21.69
Granted, Average Grant Date Fair Value	29.05
Vested, Average Grant Date Fair Value	21.69
Forfeited, Average Grant Date Fair Value	29.05
Outstanding, Average Grant Date Fair Value, Ending Balance	$ 29.05

Share-Based Compensation - Su49

Share-Based Compensation - Summary of Share-based Compensation Expense Classified in Consolidated Statements of Operations (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Share-based compensation expense	$ 12,143	$ 6,847	$ 4,024
Research and Development Expenses [Member]
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Share-based compensation expense	7,670	4,936	2,268
General and Administrative Expenses [Member]
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Share-based compensation expense	$ 4,473	$ 1,911	$ 1,756

Commitments And Contingencies -

Commitments And Contingencies - Additional Information (Detail) - USD ($)	Sep. 26, 2016	Apr. 30, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Commitments And Contingencies [Line Items]
Issuance of letter of credit recorded as restricted cash			$ 3,610,000	$ 3,601,000
Tenant improvement allowances received			(745,000)
Lease rent expense			5,600,000	1,500,000	$ 500,000
Laboratory Equipment [Member]
Commitments And Contingencies [Line Items]
Capital lease agreement period		3 years
Capital assets purchased			300,000	300,000
Capital leases accumulated amortization			$ 100,000	100,000
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
Lease extension period	5 years
Tenant improvement allowances			$ 11,500,000
Tenant improvement allowances received			3,600,000
2016 Lexington, MA Lease [Member] | Letter of Credit [Member]
Commitments And Contingencies [Line Items]
Issuance of letter of credit recorded as restricted cash			2,600,000
2015 Cambridge, MA Lease [Member]
Commitments And Contingencies [Line Items]
Lease agreement term		7 years 6 months
Lease extension period		5 years
2015 Cambridge, MA Lease [Member] | Letter of Credit [Member]
Commitments And Contingencies [Line Items]
Issuance of letter of credit recorded as restricted cash			$ 1,000,000	$ 1,000,000
2015 Boston, MA Lease Termination [Member]
Commitments And Contingencies [Line Items]
Guarantee agreement termination date			Aug. 31, 2019
2015 Boston, MA Lease Termination [Member] | Shin Nippon Biomedical Laboratories Ltd. (SNBL) [Member]
Commitments And Contingencies [Line Items]
Lease agreement term			3 years 6 months
Accelerated depreciation					200,000
Additional exit costs related to termination of sublease agreement					$ 100,000
Maximum amount of guarantee for sublease			$ 600,000

Commitments And Contingencies51

Commitments And Contingencies - Future Minimum Lease Payments Under Operating Leases (Detail) $ in Thousands	Dec. 31, 2017USD ($)
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2,018	$ 4,666
2,019	5,675
2,020	5,846
2,021	6,021
2,022	6,201
Thereafter	26,163
Total	$ 54,572

Net Loss Per Ordinary Share - S

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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net loss attributable to ordinary shareholders									$ (102,035)	$ (55,401)	$ (19,200)
Denominator:
Weighted-average ordinary shares outstanding									26,513,382	22,800,628	10,501,455
Net loss per share, basic and diluted	$ (1.09)	$ (0.94)	$ (0.92)	$ (0.89)	$ (0.79)	$ (0.75)	$ (0.51)	$ (0.36)	$ (3.85)	$ (2.43)	$ (1.83)

Net Loss Per Ordinary Share - A

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, 2017	Dec. 31, 2016
Employee Stock Option [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive shares excluded from calculation of diluted net loss per share	3,767,130	3,577,766
Restricted Share Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive shares excluded from calculation of diluted net loss per share	154,459	22,750
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 - Components of Lo

Income Taxes - Components of Loss Before Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
Loss before income taxes	$ (101,327)	$ (54,785)	$ (19,156)
Singapore [Member]
Income Taxes [Line Items]
Loss before income taxes	(76,885)	(53,387)	(16,534)
Rest Of World [Member]
Income Taxes [Line Items]
Loss before income taxes	$ (24,442)	$ (1,398)	$ (2,622)

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Mar. 31, 2018	Dec. 31, 2014
Income Taxes [Line Items]
Income tax benefit (provision)		$ (708,000)	$ (616,000)	$ (44,000)
U.S. federal corporate tax rate		35.00%
Tax cuts and jobs act of 2017, incomplete accounting, change in tax rate, deferred tax assets and liabilities, provisional income tax expense		$ 800,000
Net operating loss carryovers		28,913,000	16,046,000
Deferred tax asset			774,000
Increase in valuation allowance		20,100,000	8,500,000	4,800,000
Gross unrecognized tax benefit		6,207,000	2,343,000	1,280,000		$ 1,025,000
Unrecognized tax benefit that could affect annual effective tax rate if recognized		4,200,000
Interest and penalties related to uncertain tax positions			0
Subsidiaries Outside of Singapore [Member]
Income Taxes [Line Items]
Cash held by subsidiaries		$ 48,800,000	1,700,000
Federal [Member]
Income Taxes [Line Items]
Research and development tax credit carryforward		2,032
Federal [Member] | Research and development
Income Taxes [Line Items]
Research and development tax credit carryforward		$ 2,800,000	200,000
Federal [Member] | Orphan Drug Credits [Member]
Income Taxes [Line Items]
Research and development tax credit carryforward		$ 400,000
U.S.Orphan drug credit carryforward expiration year		2,037
State [Member]
Income Taxes [Line Items]
Research and development tax credit carryforward		2,032
State [Member] | Research and development
Income Taxes [Line Items]
Research and development tax credit carryforward		$ 1,100,000	300,000
Scenario Plan [Member]
Income Taxes [Line Items]
U.S. federal corporate tax rate	21.00%
Accounting Standards Update No. 2016-16 [Member] | Scenario Forecast [Member]
Income Taxes [Line Items]
Cumulative-effect adjustment to retained earnings					$ 400,000
Maximum [Member]
Income Taxes [Line Items]
Interest and penalties related to uncertain tax positions		100,000
United States [Member]
Income Taxes [Line Items]
Income tax benefit (provision)		$ (700,000)	(600,000)
Tax years open to examination		2,012
United States [Member] | Maximum [Member]
Income Taxes [Line Items]
Income tax benefit (provision)				(100,000)
Japan [Member]
Income Taxes [Line Items]
Income tax benefit (provision)		$ 0	0	0
Net operating loss carryovers		$ 4,100,000	5,300,000
Operating loss carryforwards expiration year		2,021
Tax years open to examination		2,008
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		$ 149,200,000	$ 84,000,000
Tax years open to examination		2,012
Ireland [Member]
Income Taxes [Line Items]
Income tax benefit (provision)		$ 0	$ 0
UK [Member]
Income Taxes [Line Items]
Income tax benefit (provision)		0
Net operating loss carryovers		$ 10,500,000

Income Taxes - Deferred Compone

Income Taxes - Deferred Components of Benefit (Provision) for Income Taxes (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current benefit (provision) for income taxes:
Total current benefit (provision) income taxes	$ 66,000	$ (1,180,000)	$ (8,000)
Deferred benefit (provision) for income taxes:
Total deferred benefit (provision) income taxes	(774,000)	564,000	(36,000)
Total benefit (provision) for income taxes	(708,000)	(616,000)	(44,000)
Singapore [Member]
Current benefit (provision) for income taxes:
Total current benefit (provision) income taxes	199,000
Deferred benefit (provision) for income taxes:
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	(133,000)	(1,180,000)	(8,000)
Deferred benefit (provision) for income taxes:
Total deferred benefit (provision) income taxes	$ (774,000)	$ 564,000	$ (36,000)

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of Statutory Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
Singapore statutory income tax rate	35.00%
Singapore [Member]
Income Taxes [Line Items]
Singapore statutory income tax rate	17.00%	17.00%	17.00%
Federal and state tax credits	5.70%	3.10%	2.30%
Permanent differences	(2.60%)	(0.90%)	5.50%
Changes in reserves for uncertain tax positions	(3.50%)	(3.60%)	(1.20%)
Foreign rate differential	2.80%	(0.10%)	1.20%
Tax rate change	(0.90%)
Other	0.50%	(0.90%)	0.20%
Change in deferred tax asset valuation allowance	(19.70%)	(15.70%)	(25.20%)
Effective income tax rate	(0.70%)	(1.10%)	(0.20%)

Income Taxes - Components of De

Income Taxes - Components of Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred tax assets:
Net operating loss carryforwards	$ 28,913	$ 16,046
Federal and state tax credits	4,522	449
Accrued expenses	1,903	242
Share-based compensation	1,921	1,024
Other	176	102
Total deferred tax assets	37,435	17,863
Valuation allowance	(36,069)	(15,999)	$ (7,466)
Net deferred tax assets	1,366	1,864
Deferred tax liabilities:
Depreciation	(1,366)	(1,090)
Total deferred tax liabilities	$ (1,366)	(1,090)
Net deferred tax assets (liabilities)		$ 774

Income Taxes - Summary of Valua

Income Taxes - Summary of Valuation Allowance (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Deferred Tax Assets Net Of Valuation Allowance [Abstract]
Balance at beginning of year	$ 15,999	$ 7,466
Increase in valuation allowance	20,595	8,774
Reversal of valuation allowance	(598)	(282)
Effect of foreign currency translation	73	41
Balance at end of year	$ 36,069	$ 15,999

Income Taxes - Summary of Activ

Income Taxes - Summary of Activity in Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation Of Unrecognized Tax Benefits Excluding Amounts Pertaining To Examined Tax Returns Roll Forward
Unrecognized tax benefit at the beginning of the year	$ 2,343	$ 1,280	$ 1,025
Tax positions released related to prior years		(1,066)
Tax positions related to the current year	3,864	2,129	255
Unrecognized tax benefit at the end of the year	$ 6,207	$ 2,343	$ 1,280

Employee Benefit Plans - Additi

Employee Benefit Plans - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
401(k) Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Discretionary contributions to defined contribution pension plan	$ 400	$ 0	$ 0

Related Parties - Additional In

Related Parties - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Third-Party Investor [Member]
Related Party Transaction [Line Items]
Related party transaction amount	$ 100	$ 100
Shin Nippon Biomedical Laboratories Ltd. (SNBL) [Member]
Related Party Transaction [Line Items]
Payment for contract research services	$ 500	$ 400	$ 100
Scientific Advisor [Member] | Consulting Agreement [Member]
Related Party Transaction [Line Items]
Consulting agreement termination notice period	14 days
Consulting service expenses	$ 13

Geographic Data - Summary of Lo

Geographic Data - Summary of Long-Lived Assets by Geographical Areas (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Geographic Area Information [Line Items]
Property and equipment, net	$ 27,334	$ 8,607
Japan [Member]
Geographic Area Information [Line Items]
Property and equipment, net	14	136
United States [Member]
Geographic Area Information [Line Items]
Property and equipment, net	$ 27,320	$ 8,471

Selected Quarterly Financial 64

Selected Quarterly Financial Information (Unaudited) - Summary of Selected Quarterly Results from Operations (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Selected Quarterly Financial Information [Abstract]
Revenues	$ 1,676	$ 676	$ 676	$ 676	$ 676	$ 392	$ 417		$ 3,704	$ 1,485	$ 152
Operating expenses	32,256	27,668	25,770	20,590	19,180	17,625	12,055	$ 7,952	106,284	56,812	19,450
Loss from operations	(30,580)	(26,992)	(25,094)	(19,914)	(18,504)	(17,233)	(11,638)	(7,952)	(102,580)	(55,327)	(19,298)
Net loss	$ (30,211)	$ (26,135)	$ (24,693)	$ (20,996)	$ (18,454)	$ (17,535)	$ (11,565)	$ (7,847)	$ (102,035)	$ (55,401)	$ (19,200)
Basic and diluted net loss per ordinary share	$ (1.09)	$ (0.94)	$ (0.92)	$ (0.89)	$ (0.79)	$ (0.75)	$ (0.51)	$ (0.36)	$ (3.85)	$ (2.43)	$ (1.83)

Subsequent Events - Additional

Subsequent Events - Additional Information (Detail) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	Feb. 28, 2018USD ($)Subsidiary$ / sharesshares	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Subsequent Event [Line Items]
Equity investment aggregate purchase price		$ 93,509	$ 30,000	$ 11,631
Collaboration And License Agreement [Member] | Takeda [Member]
Subsequent Event [Line Items]
Global strategic collaboration entered date		Feb. 28, 2018
Number of subsidiaries entered into global strategic collaboration | Subsidiary	2
Collaboration And License Agreement [Member] | Takeda [Member] | Subsequent Events [Member]
Subsequent Event [Line Items]
Shares issued under equity agreement | shares	1,096,892
Purchase price per share | $ / shares	$ 54.70
Equity investment aggregate purchase price	$ 60,000
Expected closing date of equity investment	Mar. 31, 2018