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Stock-Based Compensation - Additional Information (Detail) - USD ($)
$ / shares in Units, $ 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]      
Incentive Plan, common stock available for issuance 4,600,000    
Unrecognized compensation cost, weighted-average recognition period 1 year 8 months 12 days    
Incentive Plan, options vesting period 3 years    
Incentive Plan, options maturity period 10 years    
Incentive Plan, weighted-average grant date fair value of stock options granted $ 14.14 $ 13.49 $ 12.70
Unrecognized compensation cost related to unvested option $ 5.1    
Fair value of options vested 6.7 $ 6.8 $ 6.0
Intrinsic value of stock options exercised $ 8.7 70.6 88.1
Performance Condition Achievement      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of performance share awards vested (218,912)    
Restricted Stock Units      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Compensation costs classified as liability $ 0.9    
Unrecognized pre-tax compensation cost 17.1    
Fair value of performance share awards vested $ 22.2 $ 20.3 $ 16.4
Stock Option Awards      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Unrecognized compensation cost, weighted-average recognition period 1 year 7 months 6 days    
Performance Awards      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Unrecognized pre-tax compensation cost $ 2.6    
Unrecognized compensation cost, weighted-average recognition period 1 year 6 months    
Fair value of performance share awards vested $ 13.8    
Number of performance share awards vested (136,822)    
Director Awards      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock awards issued 19,109 15,311 16,471
Common stock issued to outside directors $ 54.93 $ 63.43 $ 57.37