Revenue from Contracts with Customers - Schedule of Impact of New Revenue Guidance on Financial Statement Line Items (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Condensed Consolidated Statement of Income
License fee	$ 379	$ 305	$ 1,138	$ 915
Type of revenue [extensible list]	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember
Net loss	$ (17,338)	$ (15,088)	$ (55,124)	$ (41,699)
Comprehensive loss	(17,284)	$ (15,096)	(55,016)	$ (41,727)
Condensed Consolidated Balance Sheet
Current portion of deferred revenue	1,517		1,517			$ 1,573
Deferred revenue, less current portion	15,029		15,029			16,759
Accumulated deficit	(420,586)		(420,586)			$ (366,111)
Accounting Standards Update 2014-09 [Member]
Condensed Consolidated Balance Sheet
Current portion of deferred revenue					$ 1,517
Deferred revenue, less current portion					16,166
Accumulated deficit					$ (365,462)
Accounting Standards Update 2014-09 [Member] | Pro Forma had the Previous Accounting Been in Effect [Member]
Condensed Consolidated Statement of Income
License fee	$ (14)		$ (42)
Type of revenue [extensible list]	us-gaap:LicenseMember		us-gaap:LicenseMember
Net loss	$ (14)		$ (42)
Comprehensive loss	(14)		(42)
Condensed Consolidated Balance Sheet
Current portion of deferred revenue	(56)		(56)
Deferred revenue, less current portion	(551)		(551)
Accumulated deficit	$ 607		$ 607