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Stock-Based Compensation Plans - Additional Information (Detail) - USD ($)
$ in Millions
9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock option compensation $ 3.0 $ 3.4
Stock Options Plan [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Number exercisable and fully vested 40,614  
Fully vested stock options with performance conditions attached 600  
Stock option compensation $ 3.0 3.4
Total intrinsic value exercised 2.6 2.0
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 4 months 17 days  
Stock Equivalent Units [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Fully vested stock options with performance conditions attached 27,655  
Total intrinsic value exercised $ 2.4 1.1
Expected compensation cost recognized over the weighted-average period 1 year 10 months 9 days  
Number of exercisable SEUs 30,360  
Compensation cost $ 7.9 $ 3.6