Pension and Other Post-Retirement Benefits - Pension Obligations, Plan Assets, and Other Post-retirement Obligations (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Accumulative other comprehensive loss:
After-tax amount recognized in accumulated other comprehensive loss	$ 76.7		$ 79.2
Defined Benefit Pension [Member]
Change in Benefit Obligation
Service cost	2.2	$ 2.0
Interest cost	2.0	3.1
Accumulative other comprehensive loss:
Defined Benefit Plan, Expected Return (Loss) on Plan Assets	(4.7)	(5.1)
Defined Benefit Plan, Amortization of Gain (Loss)	3.4	3.1
Retiree Health and Life [Member]
Change in Benefit Obligation
Service cost	0.0	0.0
Interest cost	0.1	0.1
Accumulative other comprehensive loss:
Defined Benefit Plan, Expected Return (Loss) on Plan Assets	0.0	0.0
Defined Benefit Plan, Amortization of Gain (Loss)	$ (0.1)	$ 0.0