Summary of Significant Accounting Policies - Summary of Impact of Adoption to Consolidated Statement of Income and Balance Sheet (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Collaboration revenue	$ 7,677	$ 5,917	$ 15,146	$ 12,132
Type of Revenue [Extensible List]	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Operating loss	$ (23,595)	$ (16,017)	$ (46,025)	$ (28,965)
Net loss	$ (22,219)	$ (15,593)	$ (43,575)	$ (28,224)
Net loss per share, basic and diluted	$ (0.52)	$ (0.45)	$ (1.03)	$ (0.81)
Liabilities:
Deferred revenue - current	$ 15,678		$ 15,678			$ 21,188
Deferred revenue - noncurrent	35,181		35,181			44,111
Stockholders' Equity:
Accumulated deficit	(159,257)		(159,257)			$ (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	$ 8,681		$ 16,355
Type of Revenue [Extensible List]	us-gaap:LicenseAndServiceMember		us-gaap:LicenseAndServiceMember
Operating loss	$ (22,591)		$ (44,816)
Net loss	$ (21,215)		$ (42,366)
Net loss per share, basic and diluted	$ (0.50)		$ (1.00)
Liabilities:
Deferred revenue - current	$ 18,575		$ 18,575
Deferred revenue - noncurrent	36,506		36,506
Stockholders' Equity:
Accumulated deficit	(163,479)		(163,479)
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	$ (1,004)		$ (1,209)
Type of Revenue [Extensible List]	us-gaap:LicenseAndServiceMember		us-gaap:LicenseAndServiceMember
Operating loss	$ (1,004)		$ (1,209)
Net loss	$ (1,004)		$ (1,209)
Net loss per share, basic and diluted	$ (0.02)		$ (0.03)
Liabilities:
Deferred revenue - current	$ (2,897)		$ (2,897)		(2,769)
Deferred revenue - noncurrent	(1,325)		(1,325)		(2,662)
Stockholders' Equity:
Accumulated deficit	$ 4,222		$ 4,222		$ 5,431