Stock-based Compensation - Stock-based Compensation Expense Recognized in the Consolidated Statements of Operations (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total (includes employee and non-employee stock compensation)	$ 13,145	$ 11,616	$ 5,230
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total (includes employee and non-employee stock compensation)	6,661	5,906	1,845
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total (includes employee and non-employee stock compensation)	$ 6,484	$ 5,710	$ 3,385