Share-Based Payments - Stock-Based Compensation Expense for Employee and Non-Employee Stock Options, Restricted Stock Units, Performance-Based Stock Options, Performance-Based Stock Units and Employee Stock Purchase Plan Shares (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	$ 12,187	$ 10,996	$ 22,921	$ 20,103
Research and development expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	8,190	6,561	15,215	12,089
General and administrative expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	$ 3,997	$ 4,435	$ 7,706	$ 8,014