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+ Details

Discontinued Operations - Patriot Related Matters (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	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]
Income (loss) from discontinued operations, net of income taxes	$ (24,300,000)	$ 2,300,000	$ (20,000,000.0)	$ 6,800,000
Accounts receivable, credit loss expense (reversal)	100,000	$ 0	100,000	$ 0
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							$ 150,000,000
Potential exposure from patriot bankruptcy	$ 89,700,000		$ 89,700,000		$ 90,100,000