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Stock-Based Compensation - Pre-Tax Stock-Based Compensation Expense from Continuing Operations (Detail) - USD ($)
$ 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]      
Stock-based compensation $ 43.6 $ 32.0 $ 27.6
Tax benefit 15.2 11.4 9.9
Total after tax expense 28.4 20.6 17.7
Stock Option Awards      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock-based compensation 7.4 7.2 7.4
Restricted Stock Units      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock-based compensation 21.6 17.2 13.4
Performance Awards      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock-based compensation 13.6 6.7 5.9
Director Awards      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock-based compensation $ 1.0 $ 0.9 $ 0.9