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Discontinued Operations - Patriot Related Matters (Details) $ in Millions		3 Months Ended
	Jan. 25, 2017 USD ($)	Mar. 31, 2020 USD ($) retirees	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)	Oct. 09, 2015 buyers	Dec. 31, 2007 USD ($)	Oct. 31, 2007 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Retirees | retirees		0
Estimated fund obligation							$ 40.0
Loss from discontinued operations, net of income taxes		$ (2.2)	$ (3.4)
Amount contributed to the Combined Benefit Fund		0.4	0.5
CombinedBenefitFundLowerEstimate		1.0
CombinedBenefitFundFutureEstimate		2.0
Combined Benefit Fund Liability		14.9		$ 15.2
Combined benefit fund [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Loss from discontinued operations, net of income taxes		0.1	0.2
Patriot
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Buyers | buyers					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		85.7	$ 85.7
Spinoff | UMWA 1974 Pension Plan (UMWA Plan) Litigation | Patriot
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
Litigation settlement, liability		$ 26.7		$ 26.0

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