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Stock-Based Compensation - Summary of Activity Relating to Stock Options (Detail) - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Ending balance, Outstanding Shares 7,866,789  
Stock Options [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Beginning balance, Outstanding Shares 4,231,807  
Granted, Shares 2,360,933  
Exercised, Shares (765,950)  
Forfeited, Shares (240,197)  
Ending balance, Outstanding Shares 5,586,593 4,231,807
Vested and expected to vest, Shares 4,504,257  
Exercisable, Shares 2,045,229  
Beginning balance, Outstanding Weighted Average Exercise Price $ 29.99  
Granted, Weighted Average Exercise Price 60.59  
Exercised, Weighted Average Exercise Price 20.94  
Forfeited, Weighted Average Exercise Price 43.61  
Ending balance, Outstanding Weighted Average Exercise Price 43.58 $ 29.99
Vested and expected to vest, Weighted Average Exercise Price 42.22  
Exercisable, Weighted Average Exercise Price $ 27.95  
Outstanding, Weighted Average Remaining Life 8 years 1 month 2 days 8 years 2 months 26 days
Vested and expected to vest, Weighted Average Remaining Life 7 years 11 months 23 days  
Exercisable, Weighted Average Remaining Life 6 years 11 months 26 days  
Outstanding, Aggregate Intrinsic Value $ 676,717 $ 92,843
Vested and expected to vest, Aggregate Intrinsic Value 551,729  
Exercisable, Aggregate Intrinsic Value $ 279,699