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Stock-Based Compensation Stock Based Compensation (Details) - 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]      
Pretax compensation expense $ 48.6 $ 45.3 $ 35.2
Income tax benefit (16.0) (15.1) (11.8)
Total stock-based compensation expense 32.6 30.2 23.4
Stock Awards      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Pretax compensation expense 29.4 28.1 22.5
Income tax benefit (9.6) (9.3) (7.5)
Total stock-based compensation expense 19.8 18.8 15.0
Stock Options      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Pretax compensation expense 19.2 17.2 12.7
Income tax benefit (6.4) (5.8) (4.3)
Total stock-based compensation expense $ 12.8 $ 11.4 $ 8.4