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Employee Benefit Plans - Vested RSUs - Additional Information (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Options granted 0 0 0
RSUs [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Fair value of RSUs vested during the year $ 42,740 $ 38,340 $ 32,390
Aggregate intrinsic value of RSUs vested and expected to vest $ 62,700    
Number of RSUs vested and expected to vest 2,100,000    
Unrecognized compensation expense other than options $ 34,300    
Weighted average period of recognition of unrecognized compensation cost 1 year 1 month 24 days    
Performance-based Stock Options [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Non-vested performance-based RSUs 2.00%    
Options granted 0 0 0
Market-based Stock Options [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Options granted 0 0 0