Other Postretirement Benefit Plans - Balance Sheet Items (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Amounts recognized on the Consolidated Balance Sheets:
Non-current assets	$ 2.0	$ 3.4
Accrued postretirement liability
Amounts recognized on the Consolidated Balance Sheets:
Non-current assets	0.0	36.6
Current liabilities	(5.2)	(3.8)
Non-current liabilities	(41.3)	(31.8)
Total	$ (46.5)	$ 1.0