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Share-Based Compensation Plans - Compensation Expense (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	$ 6.9	$ 32.7	$ 42.7	$ 28.2
Income tax benefit		(7.4)	(9.2)	(6.5)
Share-based payment expense, net of tax		25.3	33.5	21.7
Share-based Payment Arrangement, Cash Used to Settle Award		1.4
Fair value of shares vested		41.3	46.8	44.4
Income tax benefit realized		9.0	10.2	9.9
Performance Shares Units PSU
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense		20.2	25.8	15.8
Unrecognized Expense for Nonvested Awards at		$ 17.7
Weighted-average No. of Years Unrecognized Expense to be Recognized		1 year 6 months
Fair value of shares vested		$ 33.3	28.7	25.3
Market Share Units MSU
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense		0.0	0.0	0.1
Unrecognized Expense for Nonvested Awards at		0.0
Fair value of shares vested		0.0	0.0	8.2
Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense		6.0	6.6	6.6
Unrecognized Expense for Nonvested Awards at		$ 9.4
Weighted-average No. of Years Unrecognized Expense to be Recognized		1 year 8 months 12 days
Fair value of shares vested		$ 6.9	11.8	8.0
Deferred Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense		1.2	1.2	1.2
Unrecognized Expense for Nonvested Awards at		$ 0.3
Weighted-average No. of Years Unrecognized Expense to be Recognized		3 months 18 days
Fair value of shares vested		$ 0.6	0.9	0.7
Performance-based Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense		2.3	8.1	4.5
Unrecognized Expense for Nonvested Awards at		$ 0.3
Weighted-average No. of Years Unrecognized Expense to be Recognized		1 month 6 days
Fair value of shares vested		$ 0.5	5.4	0.0
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense		1.6	1.0	0.0
Unrecognized Expense for Nonvested Awards at		$ 1.8
Weighted-average No. of Years Unrecognized Expense to be Recognized		1 year 7 months 6 days
Fair value of shares vested		$ 0.0	$ 0.0	$ 2.2
Cash based awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized Expense for Nonvested Awards at		$ 1.3
Weighted-average No. of Years Unrecognized Expense to be Recognized		1 year 10 months 24 days