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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, 2019
USD ($)
$ / shares
shares
Dec. 31, 2018
USD ($)
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Ending balance, Outstanding Shares 11,045,846  
Stock Options [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Beginning balance, Outstanding Shares 7,530,767  
Granted, Shares 1,355,931  
Exercised, Shares (745,558)  
Forfeited, Shares (281,850)  
Ending balance, Outstanding Shares 7,859,290 7,530,767
Exercisable, Shares 3,321,049  
Beginning balance, Outstanding Weighted Average Exercise Price | $ / shares $ 93.22  
Ending balance, Outstanding Weighted Average Exercise Price | $ / shares 107.81 $ 93.22
Exercisable, Weighted Average Exercise Price | $ / shares $ 70.02  
Outstanding, Weighted Average Remaining Life 7 years 11 months 23 days 8 years 10 days
Exercisable, Weighted Average Remaining Life 6 years 10 months 20 days  
Outstanding, Aggregate Intrinsic Value | $ $ 600,965 $ 227,447
Exercisable, Aggregate Intrinsic Value | $ $ 378,713