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Stock-Based Compensation Plans - Additional Information (Detail) - USD ($)
$ in Millions
6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock option compensation $ 2.2 $ 1.9
Stock Options Plan [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Number exercisable and fully vested 44,543  
Fully vested stock options with performance conditions attached 9,421  
Stock option compensation $ 2.2 1.9
Total intrinsic value exercised 2.0 1.6
Total compensation cost related to non-vested stock options not yet recognized $ 7.9  
Expected compensation cost recognized over the weighted-average period 2 years 2 months 16 days  
Stock Equivalent Units [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Fully vested stock options with performance conditions attached 49,141  
Total intrinsic value exercised $ 1.1 1.2
Expected compensation cost recognized over the weighted-average period 2 years 1 month 20 days  
Number of exercisable SEUs 54,509  
Compensation cost $ 2.5 $ 0.0