Summary of Stock-Based Compensation Expense by Function Included within Condensed Consolidated Statement of Operations and Comprehensive Loss (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense	$ 14,156	$ 4,342
Research and Development Expenses [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense	2,446	1,873
General and Administrative Expenses [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense	$ 11,710	$ 2,469