Pension and Other Post-Retirement Benefits - Components of Net Periodic Cost (Benefit) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Defined Benefit Pension [Member]
Components of pension and other post retirement benefit costs
Service cost	$ 2.2	$ 2.0
Interest cost	2.0	3.1
Expected return on plan assets	(4.7)	(5.1)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)	2.9	3.1
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	0.0
Defined Benefit Plan, Amortization of Gain (Loss)	3.4	3.1
Retiree Health and Life [Member]
Components of pension and other post retirement benefit costs
Service cost	0.0	0.0
Interest cost	0.1	0.1
Expected return on plan assets	0.0	0.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)	0.0	0.1
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	0.0	0.0
Defined Benefit Plan, Amortization of Gain (Loss)	(0.1)	0.0
RRPF
Components of pension and other post retirement benefit costs
Equity Method Investment. Summarized Financial Data. Net Income	12.6	32.3
Alpha Partners Leasing Limited [Domain]
Defined Benefit Plan Disclosure [Line Items]
Equity Method Investment, Summarized Financial Information. Revenue.	93.8	104.0
Components of pension and other post retirement benefit costs
Equity Method Investment, Summarized Financial Information, Expenses	(87.3)	(86.4)
Equity Method Investment, Summarized Financial Information, Gains (Losses)	$ 9.0	$ 22.3