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Stock-Based Compensation - Summary of Activity Relating to Stock Options (Detail)
$ / shares in Units, $ in Thousands
6 Months Ended 12 Months Ended
Jun. 30, 2018
USD ($)
$ / shares
shares
Dec. 31, 2017
USD ($)
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Ending balance, Outstanding Shares 9,030,623  
Stock Options [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Beginning balance, Outstanding Shares 5,586,593  
Granted, Shares 2,313,844  
Exercised, Shares (490,050)  
Forfeited, Shares (54,529)  
Ending balance, Outstanding Shares 7,355,858 5,586,593
Exercisable, Shares 2,439,246  
Beginning balance, Outstanding Weighted Average Exercise Price | $ / shares $ 43.58  
Granted, Weighted Average Exercise Price | $ / shares 171.26  
Exercised, Weighted Average Exercise Price | $ / shares 36.37  
Forfeited, Weighted Average Exercise Price | $ / shares 74.88  
Ending balance, Outstanding Weighted Average Exercise Price | $ / shares 83.99 $ 43.58
Exercisable, Weighted Average Exercise Price | $ / shares $ 34.23  
Outstanding, Weighted Average Remaining Life 8 years 3 months 3 days 8 years 1 month 2 days
Exercisable, Weighted Average Remaining Life 6 years 10 months 20 days  
Outstanding, Aggregate Intrinsic Value | $ $ 575,909 $ 676,717
Exercisable, Aggregate Intrinsic Value | $ $ 298,310