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+ Details

Discontinued Operations - Patriot Related Matters (Details) $ in Millions	3 Months Ended
	Mar. 31, 2021 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)	Oct. 09, 2015 buyer	Dec. 31, 2007 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Loss from discontinued operations, net of income taxes	$ 2.0	$ 2.2
Patriot
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of buyers \| buyer				2
Spinoff \| Patriot \| Black Lung Occupational Disease Liability
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Initially determined potential exposure from patriot bankruptcy					$ 150.0
Potential exposure from patriot bankruptcy	$ 89.8		$ 90.1