Stock-Based Compensation Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award
Allocated Share-based Compensation Expense, Before Tax	$ 26.3	$ 29.5	$ 21.9
Allocated Share-based Compensation Expense, Net of Tax	20.9	19.5	14.4
Performance Share Units
Share-based Compensation Arrangement by Share-based Payment Award
Allocated Share-based Compensation Expense, Before Tax	6.8	11.5	6.6
Restricted Stock Units and Cash-Settled Awards
Share-based Compensation Arrangement by Share-based Payment Award
Allocated Share-based Compensation Expense, Before Tax	19.0	17.5	14.9
Other Awards
Share-based Compensation Arrangement by Share-based Payment Award
Allocated Share-based Compensation Expense, Before Tax	$ 0.5	$ 0.5	$ 0.4