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Stock-Based Compensation Plans - Additional Information (Detail) - USD ($)
$ in Millions
9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock option compensation $ 4.4 $ 4.9
Stock Options Plan [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Number exercisable and fully vested 35,157  
Fully vested stock options with performance conditions attached 7,611  
Stock option compensation $ 4.4 4.9
Total intrinsic value exercised 4.7 2.7
Total compensation cost related to non-vested stock options not yet recognized $ 8.4  
Expected compensation cost recognized over the weighted-average period 1 year 9 months  
Stock Equivalent Units [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Fully vested stock options with performance conditions attached 46,614  
Total intrinsic value exercised $ 6.3 6.8
Expected compensation cost recognized over the weighted-average period 2 years 18 days  
Number of exercisable SEUs 53,146  
Compensation cost $ 4.9 $ 14.0