Pension and Other Post-Retirement Benefits - Components of Net Periodic Cost (Benefit) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Pension [Member]
Components of pension and other post retirement benefit costs
Service cost	$ 8.1	$ 6.5	$ 8.2
Interest cost	12.3	15.2	14.7
Expected return on plan assets	(20.4)	(22.0)	(22.2)
Settlement expense	0.0	0.0	2.1
Amortization of unrecognized prior service credit	0.0	0.0	0.0
Unrecognized net actuarial loss (gain)	12.7	7.9	10.0
Net periodic (benefit) cost	12.7	7.6	12.8
Retiree Health and Life [Member]
Components of pension and other post retirement benefit costs
Service cost	0.2	0.2	0.2
Interest cost	0.5	0.9	1.0
Expected return on plan assets	0.0	0.0	0.0
Settlement expense	0.0	0.0	0.0
Amortization of unrecognized prior service credit	(0.2)	(0.2)	(0.1)
Unrecognized net actuarial loss (gain)	(0.4)	(0.1)	0.0
Net periodic (benefit) cost	$ 0.1	$ 0.8	$ 1.1