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Stock-Based Compensation (Stock Options) - Additional Information (Detail) - USD ($)
shares in Millions, $ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Recognition period of unvested compensation expense 2 years 10 months 24 days    
Number of shares available for grant 3.5    
Total pre-tax intrinsic value of options exercised $ 3.4 $ 1.1 $ 0.1
Fair value of options fully vested 4.3 $ 5.7 $ 6.6
Non Vested Stock Options [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Compensation cost related to non-vested stock options not yet recognized $ 3.2    
Recognition period of unvested compensation expense 1 year 6 months