Summary of Share-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, 2014	Mar. 31, 2013
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense	$ 4,342	$ 1,671
Research and Development Expenses [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense	1,873	530
General and Administrative Expenses [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense	$ 2,469	$ 1,141