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Stock-Based Compensation Plans - Additional Information (Detail) - USD ($)
$ in Millions
6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock option compensation $ 3.2 $ 1.9
Stock Options Plan [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Number exercisable and fully vested 39,325  
Fully vested stock options with performance conditions attached 6,530  
Stock option compensation $ 3.2 1.9
Total intrinsic value exercised 2.7 2.2
Total compensation cost related to non-vested stock options not yet recognized $ 12.2  
Expected compensation cost recognized over the weighted-average period 2 years 1 month 13 days  
Stock Equivalent Units [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Number exercisable and fully vested 38,488  
Fully vested stock options with performance conditions attached 35,720  
Total intrinsic value exercised $ 7.1 2.4
Expected compensation cost recognized over the weighted-average period 2 years 2 months 12 days  
Compensation cost $ 11.8 $ 5.3