Revenue from Contracts with Customers - Schedule of Impact of New Revenue Guidance on Financial Statement Line Items (Detail) - USD ($) $ 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
Consolidated Statements of Operations and Comprehensive Loss
License fee	$ 380	$ 379	$ 379	$ 379	$ 1,193	$ 305	$ 305	$ 305	$ 1,517	$ 2,108	$ 1,220
Type of revenue [extensible list]	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember
Net loss	$ (18,837)	$ (17,338)	$ (18,388)	$ (19,398)	$ (19,104)	$ (15,088)	$ (13,639)	$ (12,971)	$ (73,961)	$ (60,802)	$ (44,472)
Comprehensive loss									(73,843)	(61,001)	$ (44,444)
Consolidated Balance Sheet
Current portion of deferred revenue	1,517				1,573				1,517	1,573
Deferred revenue, less current portion	14,650				16,759				14,650	16,759
Accumulated deficit	(439,423)				$ (366,111)				(439,423)	$ (366,111)
ASU 2014-09 [Member]
Consolidated Balance Sheet
Current portion of deferred revenue												$ 1,517
Deferred revenue, less current portion												16,166
Accumulated deficit												$ (365,462)
ASU 2014-09 [Member] | Pro Forma had the Previous Accounting Been in Effect [Member]
Consolidated Statements of Operations and Comprehensive Loss
License fee									$ (56)
Type of revenue [extensible list]									us-gaap:LicenseMember
Net loss									$ (56)
Comprehensive loss									(56)
Consolidated Balance Sheet
Current portion of deferred revenue	(56)								(56)
Deferred revenue, less current portion	(537)								(537)
Accumulated deficit	$ 593								$ 593