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Employee Incentive Plans - Additional Information (Detail) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2017
Mar. 31, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Performance Shares          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Performance period   1 year      
Vesting period 2 years        
Internally Developed Software          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Stock-based compensation expense, capitalized amount $ 2.2   $ 2.2 $ 7.4 $ 6.2
Minimum | Performance Shares          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Award issuance range   0.00%      
Maximum | Performance Shares          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Award issuance range   200.00%      
Share-based Compensation Award, Tranche One | Performance Shares          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Vesting rate 50.00%        
Share-based Compensation Award, Tranche Two | Performance Shares          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Vesting rate 50.00%