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Stock-Based Compensation - Summary of Activity Relating to Stock Options (Detail) - Stock Options [Member] - USD ($)
$ / shares in Units, $ in Thousands
9 Months Ended 12 Months Ended
Sep. 30, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Beginning balance, Outstanding Shares 4,231,807  
Granted, Shares 2,169,250  
Exercised, Shares (286,748)  
Forfeited, Shares (176,132)  
Ending balance, Outstanding Shares 5,938,177 4,231,807
Exercisable, Shares 2,301,343  
Beginning balance, Outstanding Weighted Average Exercise Price $ 29.99  
Granted, Weighted Average Exercise Price 58.33  
Exercised, Weighted Average Exercise Price 22.20  
Forfeited, Weighted Average Exercise Price 42.57  
Ending balance, Outstanding Weighted Average Exercise Price 40.35 $ 29.99
Exercisable, Weighted Average Exercise Price $ 25.30  
Outstanding, Weighted Average Remaining Life 8 years 2 months 8 days 8 years 2 months 27 days
Exercisable, Weighted Average Remaining Life 7 years 1 month 6 days  
Outstanding, Aggregate Intrinsic Value $ 140,390 $ 92,843
Exercisable, Aggregate Intrinsic Value $ 85,819