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Stock-Based Compensation - Summary of Activity Relating to Stock Options (Detail)
$ / shares in Units, $ in Thousands
9 Months Ended 12 Months Ended
Sep. 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 10,001,048  
Stock Options [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Beginning balance, Outstanding Shares 5,586,593  
Granted, Shares 2,775,594  
Exercised, Shares (711,621)  
Forfeited, Shares (155,855)  
Ending balance, Outstanding Shares 7,494,711 5,586,593
Exercisable, Shares 2,460,232  
Beginning balance, Outstanding Weighted Average Exercise Price | $ / shares $ 43.58  
Granted, Weighted Average Exercise Price | $ / shares 168.97  
Exercised, Weighted Average Exercise Price | $ / shares 35.13  
Forfeited, Weighted Average Exercise Price | $ / shares 79.46  
Ending balance, Outstanding Weighted Average Exercise Price | $ / shares 90.07 $ 43.58
Exercisable, Weighted Average Exercise Price | $ / shares $ 36.17  
Outstanding, Weighted Average Remaining Life 8 years 1 month 24 days 8 years 1 month 2 days
Exercisable, Weighted Average Remaining Life 6 years 8 months 23 days  
Outstanding, Aggregate Intrinsic Value | $ $ 459,643 $ 676,717
Exercisable, Aggregate Intrinsic Value | $ $ 258,528