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Stock-Based Compensation - Additional Information (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Restricted stock [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Fair value at vesting date $ 39.8 $ 30.0 $ 23.6
Unrecognized compensation expense related to non-vested $ 58.0    
Unrecognized compensation expense related to non-vested, period for recognition, in years 1 year 2 months    
Restricted stock [Member] | Minimum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Grants vest over periods, in years 1 year    
Restricted stock [Member] | Maximum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Grants vest over periods, in years 3 years    
2013 Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Common stock available for issue 19,680,072    
2013 Plan [Member] | Restricted stock [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock available to grant 14,538,540