Employee Benefits - Amounts Recognized on the Balance Sheets (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Pension and OPEB obligations	$ 326.2	$ 422.8
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Other long-term assets	188.8	139.1
Pension and OPEB obligations	305.5	375.5
Total net (liabilities) assets	(116.7)	(236.4)
OPEB Benefits
Defined Benefit Plan Disclosure [Line Items]
Other long-term assets	341.7	210.8
Pension and OPEB obligations	20.7	47.3
Total net (liabilities) assets	$ 321.0	$ 163.5