Stock-Based Compensation (Allocation of Stock-Based Compensation Costs) (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 [Line Items]
Stock-based compensation expense	$ 36.6	$ 44.4	$ 31.7
Income tax benefit related to the stock-based compensation expense	6.7	10.0	7.0
Restricted stock units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	22.2	20.4	19.8
Performance share awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	10.2	20.6	10.2
Market Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 4.2	$ 3.4	$ 1.7