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Stock-Based Compensation - Summary of Activity Relating to Stock Options (Detail) - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Ending balance, Outstanding Shares 9,172,948  
Stock Options [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Beginning balance, Outstanding Shares 5,586,593  
Granted, Shares 1,376,044  
Exercised, Shares (347,725)  
Forfeited, Shares (33,623)  
Ending balance, Outstanding Shares 6,581,289 5,586,593
Exercisable, Shares 2,163,375  
Beginning balance, Outstanding Weighted Average Exercise Price $ 43.58  
Granted, Weighted Average Exercise Price 187.51  
Exercised, Weighted Average Exercise Price 32.65  
Forfeited, Weighted Average Exercise Price 62.08  
Ending balance, Outstanding Weighted Average Exercise Price 74.16 $ 43.58
Exercisable, Weighted Average Exercise Price $ 31.70  
Outstanding, Weighted Average Remaining Life 8 years 3 months 14 days 8 years 1 month 2 days
Exercisable, Weighted Average Remaining Life 7 years  
Outstanding, Aggregate Intrinsic Value $ 608,335 $ 676,717
Exercisable, Aggregate Intrinsic Value $ 279,868