Annual report pursuant to Section 13 and 15(d)

Equity Incentive Plans and Stock Based Compensation - Stock-Based Compensation Expense (Detail)

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Equity Incentive Plans and Stock Based Compensation - Stock-Based Compensation Expense (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]      
Stock-based compensation expense $ 6,801 $ 5,488 $ 4,922
Research and Development      
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]      
Stock-based compensation expense 4,090 3,123 2,562
General and Administrative      
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]      
Stock-based compensation expense $ 2,711 $ 2,365 $ 2,360