Employee Stock-based Compensation (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 29,510	$ 20,103
Impact on basic and diluted net loss per share	$ 0.91	$ 0.64
Weighted average shares (basic and diluted)	32,478,408	31,588,315
Employee Stock Option | Research and development
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 23,556	$ 15,254
Employee Stock Option | General and administrative
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	5,882	4,704
Performance Awards
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 72	$ 145