Discontinued Operations - Patriot Related Matters (Details) $ in Millions		12 Months Ended
	Jan. 25, 2017 USD ($)	Dec. 31, 2020 USD ($) retiree	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Oct. 09, 2015 buyer	Dec. 31, 2007 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of buyers | buyer					2
Loss contingency accrual, mark-to-market adjustment			$ 0.1
Amount contributed to the combined benefit fund		$ 1.6	1.9	$ 2.2
Combined benefit fund liability		13.2	15.2
Minimum
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Combined benefit fund future estimate		1.0
Maximum
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Combined benefit fund future estimate		1.5
Combined benefit fund
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Loss contingency accrual, mark-to-market adjustment		$ (0.9)		(2.4)
Number of retirees | retiree		0
Estimated fund obligation		$ 40.0
Loss from discontinued operations before income taxes		0.5	0.7	0.7
Patriot Coal Corporation | Disposal Group, Disposed of by Means Other than Sale, Not Discontinued Operations, Spinoff | UMWA1974 Pension Plan Litigation
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Settlement of claim, amount awarded to other party	$ 75.0
Estimated litigation liability		13.9	26.0
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		90.1	85.7			$ 150.0
Loss contingency accrual, mark-to-market adjustment		$ 4.2	$ (18.3)	$ (33.7)