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Stock-Based Compensation - Schedule of Stock-Based Compensation Expense (Detail) (USD $)
In Thousands, unless otherwise specified
3 Months Ended 9 Months Ended
Sep. 30, 2014
Sep. 30, 2013
Sep. 30, 2014
Sep. 30, 2013
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]        
Stock-based compensation expense $ 856 $ 137 $ 2,249 $ 394
Research and Development [Member]
       
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]        
Stock-based compensation expense 380 113 1,062 285
General and Administrative [Member]
       
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]        
Stock-based compensation expense $ 476 $ 24 $ 1,187 $ 109