Stock-Based Compensation - Aggregate Stock-Based Compensation Expense Related to the Issuance of Stock Option Awards (Detail) (USD $)
In Thousands, unless otherwise specified
3 Months Ended 6 Months Ended
Jun. 30, 2014
Jun. 30, 2013
Jun. 30, 2014
Jun. 30, 2013
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]        
Total $ 611 $ 577 $ 1,458 $ 876
Research and Development [Member]
       
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]        
Total 252 445 774 625
General and Administrative Expense [Member]
       
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]        
Total $ 359 $ 132 $ 684 $ 251