Pension and Postretirement Plans - Schedule of Pension and Postretirement Costs (Benefits) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	$ 0.0	$ 0.0
Other components of pension and postretirement benefit plans expense:
Interest cost on projected benefit obligation	3.8	4.8
Expected return on plan assets	(7.5)	(7.3)
Amortization of:
Prior service benefit	0.0	0.0
Actuarial loss	6.3	4.5
Pension / postretirement costs	2.6	2.0
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	0.1	0.1
Other components of pension and postretirement benefit plans expense:
Interest cost on projected benefit obligation	0.6	0.9
Expected return on plan assets	0.0	0.0
Amortization of:
Prior service benefit	(0.6)	(0.6)
Actuarial loss	0.0	0.2
Pension / postretirement costs	$ 0.1	$ 0.6