Stock-Based Compensation Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award
Allocated Share-based Compensation Expense, Before Tax	$ 30.4	$ 26.6	$ 26.3
Allocated Share-based Compensation Expense, Net of Tax	26.1	22.7	20.9
Performance Share Units
Share-based Compensation Arrangement by Share-based Payment Award
Allocated Share-based Compensation Expense, Before Tax	5.6	5.0	6.8
Restricted Stock Units and Cash-Settled Awards
Share-based Compensation Arrangement by Share-based Payment Award
Allocated Share-based Compensation Expense, Before Tax	23.9	21.0	19.0
Other Awards
Share-based Compensation Arrangement by Share-based Payment Award
Allocated Share-based Compensation Expense, Before Tax	0.5	0.6	0.5
Stock Success Units (SSUs)
Share-based Compensation Arrangement by Share-based Payment Award
Allocated Share-based Compensation Expense, Before Tax	$ 0.4	$ 0.0	$ 0.0