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Share-based Payments - Additional Information (Detail)
1 Months Ended 12 Months Ended
Jun. 30, 2017
$ / shares
Feb. 28, 2017
$ / shares
Sep. 30, 2016
$ / shares
Jun. 30, 2016
$ / shares
May 31, 2016
$ / shares
Feb. 29, 2016
$ / shares
Mar. 31, 2007
yr
shares
$ / shares
Dec. 31, 2017
USD ($)
yr
Dec. 31, 2016
USD ($)
yr
Dec. 31, 2015
USD ($)
Disclosure of terms and conditions of share-based payment arrangement [line items]                    
Share-based compensation expense               $ 7,400,000 $ 7,500,000 $ 4,000,000
Fair value of non-vested stock awards | $ / shares $ 107.29 $ 107.29 $ 59.94 $ 59.94 $ 63.30 $ 59.94        
Estimated compensation cost for next year               $ 9,300,000 $ 13,100,000  
Weighted average remaining contractual life | yr               2.1 2.8  
Additional compensation cost recognized               $ 0    
Share option expiration period               Mar. 31, 2017    
Vesting period               3 years    
Expected term               10 years    
2017 [member]                    
Disclosure of terms and conditions of share-based payment arrangement [line items]                    
Estimated compensation cost for next year               $ 4,900,000    
Stock options [member]                    
Disclosure of terms and conditions of share-based payment arrangement [line items]                    
Weighted average remaining contractual life | yr             10      
Equity stock options granted | shares             35,657      
Exercise price of the options | $ / shares             $ 53.1      
Weighted-average fair value of the stock options at the grant date | $ / shares             $ 22.3      
Expected dividend yield             0.58%      
Expected volatility             37.80%      
Weighted average risk-free interest rate             4.59%      
Expected term | yr             6