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Discontinued Operations - Patriot Related Matters (Details) $ in Millions		3 Months Ended		9 Months Ended
	Jan. 25, 2017 USD ($)	Sep. 30, 2019 USD ($) retirees	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) retirees	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	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		0
Estimated fund obligation									$ 40.0
Amount contributed to the Combined Benefit Fund				$ 1.4	$ 1.7
CombinedBenefitFundLowerEstimate		$ 1.0
CombinedBenefitFundFutureEstimate		2.0
Combined Benefit Fund Liability		15.4		15.4		$ 16.4
Combined benefit fund [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax		0.2	$ 0.2	0.5	$ 0.5
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		103.8		103.8		102.7
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		$ 24.9		$ 24.9		$ 36.7

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