Discontinued Operations - Patriot Related Matters (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	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]
Loss from discontinued operations, net of income taxes	$ 2,300,000	$ 2,300,000	$ 4,300,000	$ 4,500,000
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