Discontinued Operations - Patriot Related Matters (Details) $ in Millions	12 Months Ended
	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Oct. 09, 2015 buyer	Dec. 31, 2008 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of buyers | buyer				2
Patriot Coal Corporation | Black Lung Occupational Disease Liability | Disposal Group, Disposed of by Means Other than Sale, Not Discontinued Operations, Spinoff
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Loss contingency accrual	$ 87.2	$ 90.1			$ 150.0
Loss contingency accrual, mark-to-market adjustment	$ 2.1	$ 4.2	$ (18.3)