Summary of Significant Accounting Policies - Summary of Impact of Adoption to Consolidated Statement of Operations and Balance Sheet (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Collaboration revenue	$ 7,880	$ 7,408	$ 7,677	$ 7,469	$ 6,668	$ 7,317	$ 5,917	$ 6,215	$ 30,434	$ 26,117	$ 16,479
Type of Revenue [Extensible List]									us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Operating loss	(20,746)	(24,099)	(23,595)	(22,430)	(24,715)	(15,875)	(16,017)	(12,948)	$ (90,870)	$ (69,555)	$ (32,159)
Net loss	$ (19,066)	$ (22,702)	$ (22,219)	$ (21,356)	$ (23,963)	$ (15,356)	$ (15,593)	$ (12,631)	$ (85,343)	$ (67,543)	$ (31,634)
Net loss per share, basic and diluted	$ (0.43)	$ (0.53)	$ (0.52)	$ (0.51)	$ (0.61)	$ (0.44)	$ (0.45)	$ (0.36)	$ (1.98)	$ (1.88)	$ (1.42)
Liabilities:
Deferred revenue – current	$ 27,122				$ 21,188				$ 27,122	$ 21,188
Deferred revenue – noncurrent	28,810				44,111				28,810	44,111
Stockholders’ Equity:
Accumulated deficit	(201,025)				$ (121,113)				(201,025)	$ (121,113)
Accounting Standards Update (“ASU”) No. 2014-09 [Member]
Liabilities:
Deferred revenue – current												$ 18,419
Deferred revenue – noncurrent												41,449
Stockholders’ Equity:
Accumulated deficit												(115,682)
Accounting Standards Update (“ASU”) No. 2014-09 [Member] | Balances without Adoption of ASC 606 [Member]
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Collaboration revenue									$ 31,838
Type of Revenue [Extensible List]									us-gaap:LicenseAndServiceMember
Operating loss									$ (89,466)
Net loss									$ (83,939)
Net loss per share, basic and diluted									$ (1.95)
Liabilities:
Deferred revenue – current	31,149								$ 31,149
Deferred revenue – noncurrent	28,810								28,810
Stockholders’ Equity:
Accumulated deficit	(205,052)								(205,052)
Accounting Standards Update (“ASU”) No. 2014-09 [Member] | Effect of Change Higher/(Lower) [Member]
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Collaboration revenue									$ (1,404)
Type of Revenue [Extensible List]									us-gaap:LicenseAndServiceMember
Operating loss									$ (1,404)
Net loss									$ (1,404)
Net loss per share, basic and diluted									$ (0.03)
Liabilities:
Deferred revenue – current	(4,027)								$ (4,027)			(2,769)
Deferred revenue – noncurrent												(2,662)
Stockholders’ Equity:
Accumulated deficit	$ 4,027								$ 4,027			$ 5,431