Pension and Other Post-Retirement Benefits - Pension Obligations, Plan Assets, and Other Post-retirement Obligations (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Accumulative other comprehensive loss:
After-tax amount recognized in accumulated other comprehensive loss	$ 83.2		$ 85.4
Defined Benefit Pension [Member]
Change in Benefit Obligation
Service cost	2.0	$ 1.7
Interest cost	3.1	3.8
Accumulative other comprehensive loss:
Defined Benefit Plan, Expected Return (Loss) on Plan Assets	(5.1)	(5.5)
Defined Benefit Plan, Amortization of Gain (Loss)	3.1	1.9
Retiree Health and Life [Member]
Change in Benefit Obligation
Service cost	0.0	0.0
Interest cost	0.1	0.2
Accumulative other comprehensive loss:
Defined Benefit Plan, Expected Return (Loss) on Plan Assets	0.0	0.0
Defined Benefit Plan, Amortization of Gain (Loss)	$ 0.0	$ 0.0