Pensions and Other Postretirement Benefits - Net Periodic Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Benefits
Components of Net Periodic Cost:
Service Cost	$ 15.4	$ 14.0	$ 17.3
Interest Cost	14.0	46.1	41.8
Expected Return on Plan Assets	(21.0)	(54.9)	(63.6)
Amortization:
Prior Service Cost (Credit)	0.2	0.2	0.4
Actuarial Loss (Gain)	5.4	10.0	5.9
Net Curtailment/Settlement Loss	153.7	39.2	1.0
Other	0.2	0.3	0.5
Net Periodic Cost (Benefit)	167.9	54.9	3.3
Postretirement Health Coverage
Components of Net Periodic Cost:
Service Cost	0.5	0.5	0.6
Interest Cost	1.0	1.2	1.2
Expected Return on Plan Assets	0.0	0.0	0.0
Amortization:
Prior Service Cost (Credit)	(0.3)	(0.3)	(0.3)
Actuarial Loss (Gain)	(1.7)	(2.3)	(1.8)
Net Curtailment/Settlement Loss	0.0	0.0	0.0
Other	0.0	0.0	0.0
Net Periodic Cost (Benefit)	$ (0.5)	$ (0.9)	$ (0.3)