Pension and Other Postretirement Benefit Plans - Reconciliation of Net Pension Amounts Recognized in Consolidated Balance Sheet (Details) - USD ($) $ in Millions		Dec. 31, 2020	Dec. 31, 2019
Other Postretirement Benefits Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Net Loss	[1]	$ (23.0)	$ (16.0)
Prior Service Credit		(28.3)	(36.3)
Accumulated Contributions in Excess of Net Periodic Benefit Cost (Prepaid Pension Asset)		13.1	3.6
Total Net Pension Amounts Recognized in Consolidated Balance Sheet		18.4	23.9
Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Net Loss		(299.0)	(243.4)
Prior Service Credit		1.1	1.3
Accumulated Contributions in Excess of Net Periodic Benefit Cost (Prepaid Pension Asset)		91.6	87.7
Total Net Pension Amounts Recognized in Consolidated Balance Sheet		(206.3)	(154.4)
Trust for Benefit of Employees [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Long-term Investments		$ 20.4	$ 19.1

[1] Excludes gains, losses and contributions associated with irrevocable grantor trusts.