Pension and Other Post-Retirement Benefits - Pension Obligations, Plan Assets, and Other Post-retirement Obligations (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Mar. 31, 2020	Dec. 31, 2019
Accumulative other comprehensive loss:
After-tax amount recognized in accumulated other comprehensive loss	$ 79.7		$ 79.7		$ 83.2	$ 85.4
Defined Benefit Pension [Member]
Change in Benefit Obligation
Service cost	2.0	$ 1.5	4.0	$ 3.2
Interest cost	3.0	3.8	6.1	7.6
Accumulative other comprehensive loss:
Defined Benefit Plan, Expected Return (Loss) on Plan Assets	(5.0)	(5.5)	(10.1)	(11.0)
Defined Benefit Plan, Amortization of Gain (Loss)	3.3	2.1	6.4	4.0
Retiree Health and Life [Member]
Change in Benefit Obligation
Service cost	0.1	0.1	0.1	0.1
Interest cost	0.2	0.3	0.3	0.5
Accumulative other comprehensive loss:
Defined Benefit Plan, Expected Return (Loss) on Plan Assets	0.0	0.0	0.0	0.0
Defined Benefit Plan, Amortization of Gain (Loss)	$ (0.2)	$ (0.1)	$ (0.2)	$ (0.1)