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Share-Based Compensation - Additional Information (Detail) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Number of shares available for grant (in shares) 3,800,000  
Shares reserved for future issuance (in shares) 22,900,000  
Weighted average grant date value (in dollars per share) $ 169.24 $ 136.51
Employee Stock Purchase Plan    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Number of shares available for grant (in shares) 749,000  
Shares reserved for future issuance (in shares) 2,000,000  
Percentage of annual compensation participants may contribute (in percentage) 15.00%  
Purchase price as a percentage of closing price on the last day of the quarter for ESPP transactions (in percentage) 85.00%  
Stock Option Plan    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Contractual term (in years) 10 years  
Unvested options to purchase (in shares) 1,700,000 2,400,000
Total unrecognized compensation cost $ 137.6  
Weighted average amortization period (in years) 3 years  
Minimum    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Vesting period (in years) 3 years  
Minimum | Stock Option Plan    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Vesting period (in years) 3 years  
Minimum | Restricted Stock Units (RSUs)    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Vesting period (in years) 3 years  
Maximum    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Vesting period (in years) 6 years  
Maximum | Stock Option Plan    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Vesting period (in years) 6 years  
Maximum | Restricted Stock Units (RSUs)    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Vesting period (in years) 5 years