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Stock-Based Compensation Plans - Additional Information (Detail) - USD ($)
$ in Millions
9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock option compensation $ 3.4 $ 2.8
Stock Options Plan [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Number exercisable and fully vested 44,443  
Fully vested stock options with performance conditions attached 9,421  
Stock option compensation $ 3.4 2.8
Total intrinsic value exercised 2.0 1.8
Total compensation cost related to non-vested stock options not yet recognized $ 6.7  
Expected compensation cost recognized over the weighted-average period 2 years 18 days  
Stock Equivalent Units [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Fully vested stock options with performance conditions attached 49,391  
Total intrinsic value exercised $ 1.1 1.5
Expected compensation cost recognized over the weighted-average period 1 year 11 months 4 days  
Number of exercisable SEUs 54,759  
Compensation cost $ 3.6 $ 3.7