Stock Based Compensation - Summary of Stock Based Compensation (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Compensation expense relating to restricted shares	$ 9,381	$ 7,044	$ 16,058	$ 7,241
Research and Development
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Compensation expense relating to restricted shares	4,765	4,760	7,544	4,806
General and Administrative
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Compensation expense relating to restricted shares	$ 4,616	$ 2,284	$ 8,514	$ 2,435