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	$ 59.8	$ 118.5
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Other long-term assets	6.1	12.7
Pension and OPEB obligations	59.8	92.3
Total net (liabilities) assets	(53.7)	(79.6)
OPEB Benefits
Defined Benefit Plan Disclosure [Line Items]
Other long-term assets	21.2	0.0
Pension and OPEB obligations	0.0	26.2
Total net (liabilities) assets	$ 21.2	$ (26.2)