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Stock-Based Compensation - Stock-Based Compensation Expense (Detail) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]        
Total stock-based compensation expense $ 2,219 $ 2,735 $ 6,969 $ 13,128
Research and development [Member]        
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]        
Total stock-based compensation expense 1,331 1,790 3,766 5,368
General and administrative [Member]        
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]        
Total stock-based compensation expense $ 888 $ 945 $ 3,203 $ 7,760