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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 $ 32.0 $ 27.6 $ 28.1
Tax benefit 11.4 9.9 10.5
Total after tax expense 20.6 17.7 17.6
Stock Option Awards      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock-based compensation 7.2 7.4 7.8
Restricted Stock Units      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock-based compensation 17.2 13.4 11.8
Performance Awards      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock-based compensation 6.7 5.9 7.6
Director Awards      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock-based compensation $ 0.9 $ 0.9 $ 0.9