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Stock-Based Compensation - Summary of Stock-Based Compensation Expense by Award Type Recognized (Detail) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Total Stock-based compensation expense $ 15,817 $ 6,208
Stock Options [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Total Stock-based compensation expense 15,381 6,033
Restricted Stock Units [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Total Stock-based compensation expense 168 100
Employee Stock Purchase Plan [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Total Stock-based compensation expense $ 268 $ 75