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Stock-Based Compensation - Summary of Restricted Stock Unit and Performance Share Transaction (Detail) - $ / shares
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Restricted Stock Units      
Units/Shares Activity      
Outstanding as of beginning period (in shares) 572,213    
Granted (in shares) 141,439    
Distributed (vested) (in shares) (162,144)    
Cancelled (in shares) (34,177)    
Outstanding as of end of period (in shares) 517,331 572,213  
Vested or expected to vest (in shares) 517,331    
Weighted Average Grant Date Fair Value      
Outstanding as of beginning of period (in dollars per share) $ 109.31    
Granted (in dollars per share) 168.86 $ 142.14 $ 120.42
Outstanding as of end of period (in dollars per share) $ 125.30 $ 109.31  
Performance Shares      
Units/Shares Activity      
Outstanding as of beginning period (in shares) 1,199,570    
Granted (in shares) 274,026    
Distributed (vested) (in shares) (453,956)    
Cancelled (in shares) (98,934)    
Outstanding as of end of period (in shares) 920,706 1,199,570  
Vested or expected to vest (in shares) 651,244    
Weighted Average Grant Date Fair Value      
Outstanding as of beginning of period (in dollars per share) $ 115.33    
Granted (in dollars per share) 179.40 $ 132.94 $ 119.99
Outstanding as of end of period (in dollars per share) $ 139.56 $ 115.33