Employee Benefits - Amounts Recognized on the Balance Sheets (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Pension and OPEB obligations	$ 327.0	$ 326.2
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Other long-term assets	182.9	188.8
Pension and OPEB obligations	304.3	305.5
Total net (liabilities) assets	(121.4)	(116.7)
OPEB Benefits
Defined Benefit Plan Disclosure [Line Items]
Other long-term assets	418.0	341.7
Pension and OPEB obligations	22.7	20.7
Total net (liabilities) assets	$ 395.3	$ 321.0