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Redeemable Convertible Preferred Stock and Equity (Deficit) - Schedule of Allocated Stock-Based Compensation Expense (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]      
Total stock-based compensation expense $ 27,681 $ 18,698 $ 3,444
Research and development [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]      
Total stock-based compensation expense 16,987 10,893 1,925
General and administrative [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]      
Total stock-based compensation expense $ 10,694 $ 7,805 $ 1,519