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Share-Based Payments - Additional Information (Detail)
3 Months Ended 9 Months Ended
Sep. 30, 2017
USD ($)
program
Sep. 30, 2017
USD ($)
program
Sep. 30, 2016
USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
General vesting period   3 years  
Number of incentive programs | program 2 2  
Total unrecognized compensation cost related to unvested share-based payments $ 127,900,000 $ 127,900,000  
Weighted average period for unrecognized compensation cost expected to be recognized (in years)   2 years 2 months 12 days  
Proceeds from exercise of stock options   $ 1,768,000 $ 28,531,000
Stock Options | Maximum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock option expire from date of grant, period   10 years  
Performance Cycle 2017-2019 | Performance Shares      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Period for evaluation of TSR   3 years  
Share-based compensation 0 $ 0  
Performance Cycle 2017-2019 | Performance Shares Market Related Component      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based compensation 100,000 100,000  
Performance Cycle 2016-2018 | Performance Shares      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based compensation 0 0  
Performance Cycle 2016-2018 | Performance Shares Market Related Component      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based compensation $ 100,000 $ 300,000