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Stock-Based Compensation - Pre-Tax Stock-Based Compensation Expense (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock-based compensation $ 27.6 $ 28.1 $ 24.9
Tax benefit 9.9 10.5 9.1
Total after tax expense 17.7 17.6 15.8
Stock Option Awards      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock-based compensation 7.4 7.8 7.9
Restricted Stock Units      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock-based compensation 13.4 11.8 9.6
Performance Awards      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock-based compensation 5.9 7.6 6.5
Director Awards      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock-based compensation $ 0.9 $ 0.9 $ 0.9