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Stock-Based Compensation - Schedule of Allocated Stock-Based Compensation Expense (Detail) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]    
Total stock-based compensation expense $ 10,878 $ 8,409
Research and development [Member]    
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]    
Total stock-based compensation expense 6,370 4,901
General and administrative [Member]    
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]    
Total stock-based compensation expense $ 4,508 $ 3,508