Pension and Other Post-Retirement Benefits - Components of Net Periodic Cost (Benefit) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Defined Benefit Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2.1	$ 2.0	$ 6.5	$ 6.0
Interest cost	2.1	3.1	6.1	9.2
Expected return on plan assets	(4.6)	(5.0)	(13.9)	(15.1)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	0.0		0.0
Defined Benefit Plan, Amortization of Gain (Loss)	3.3	3.1	9.9	9.5
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)	2.9	3.2	8.6	9.6
Retiree Health and Life [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.0	0.0	0.1	0.1
Interest cost	0.1	0.1	0.2	0.4
Expected return on plan assets	0.0	0.0	0.0	0.0
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	0.0	0.0	(0.1)	(0.1)
Defined Benefit Plan, Amortization of Gain (Loss)	(0.1)	(0.1)	(0.2)	(0.3)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)	$ 0.0	$ 0.0	$ 0.0	$ 0.1