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Share-Based Compensation - Additional Information (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock options granted (in shares) 0    
Cash received from option exercises $ 41.6 $ 4.1 $ 27.3
Tax benefit realized from stock options exercised 4.0 1.1 4.8
Intrinsic value of options exercised 26.4 0.9 6.9
Stock options      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Unrecognized compensation cost $ 0.8    
Unrecognized compensation cost, recognition period (in years) 1 year    
Restricted Shares      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Unrecognized compensation cost $ 22.9    
Unrecognized compensation cost, recognition period (in years) 4 years 1 month    
Fair value, vested $ 10.4 9.9 10.4
Restricted Stock Units (RSUs)      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Unrecognized compensation cost $ 0.2    
Unrecognized compensation cost, recognition period (in years) 7 months 5 days    
Fair value, vested $ 1.7 1.9 2.4
Released intrinsic value 5.4 $ 4.1 $ 9.3
Performance Stock Units      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Unrecognized compensation cost $ 2.1    
Unrecognized compensation cost, recognition period (in years) 9 months 18 days    
Fair value, vested $ 1.4    
Omnibus Plan 2017      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Common stock available for grant (in shares) 1,746,500