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Stock-Based Compensation - Summary of Restricted Stock Unit and Performance Share Transaction (Detail) - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Restricted Stock Units      
Restricted Stock Units      
Outstanding beginning balance (in shares) 623,142    
Granted (in shares) 153,675    
Distributed (vested) (in shares) (194,373)    
Cancelled (in shares) (10,231)    
Outstanding at end of period (in shares) 572,213 623,142  
Vested or expected to vest (in shares) 572,213    
Weighted Average Grant Date Fair Value      
Outstanding beginning balance (in dollars per share) $ 92.54    
Granted (in dollars per share) 142.14 $ 120.42 $ 104.46
Outstanding ending balance (in dollars per share) $ 109.31 $ 92.54  
Performance Shares      
Restricted Stock Units      
Outstanding beginning balance (in shares) 1,455,054    
Granted (in shares) 389,384    
Distributed (vested) (in shares) (571,254)    
Cancelled (in shares) (73,614)    
Outstanding at end of period (in shares) 1,199,570 1,455,054  
Vested or expected to vest (in shares) 1,150,611    
Weighted Average Grant Date Fair Value      
Outstanding beginning balance (in dollars per share) $ 94.19    
Granted (in dollars per share) 132.94 $ 119.99 $ 97.92
Outstanding ending balance (in dollars per share) $ 115.33 $ 94.19