Discontinued Operations - Patriot Related Matters (Details) - USD ($) $ in Millions		3 Months Ended
	Jan. 25, 2017	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2007	Oct. 31, 2007
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Estimated fund obligation						$ 40.0
Amount contributed to the Combined Benefit Fund		$ 0.5	$ 0.6
CombinedBenefitFundLowerEstimate		1.0
CombinedBenefitFundFutureEstimate		2.0
Combined Benefit Fund Liability		16.0		$ 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
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		103.1		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		$ 37.8		$ 36.7