Discontinued Operations Textuals (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Apr. 01, 2017	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2015	Oct. 31, 2007
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Disposal Group, Including Discontinued Operation, Revenue			$ 0.0	$ 0.0	$ 0.0	$ 0.0
Potential exposure from Patriot bankruptcy								$ 150.0
Patriot Bankruptcy Liability	$ 85.7	$ 102.7			85.7	102.7
Net mark-to-market adjustment on actuarially determined liabilities	(67.4)	(125.5)	0.0	45.2	(67.4)	125.5
Estimated fund obligation								$ 40.0
Amount contributed to the Combined Benefit Fund			0.6	1.7	1.9	2.2
Combined Benefit Fund Lower Estimate					1.0
Combined Benefit Fund Future Estimate					2.0
Combined Benefit Fund Liability	$ 15.2	$ 16.4			15.2	16.4
Payment to the VEBA based on the construct of the negotiated settlement							$ 75.0
Income (loss) from discontinued operations, net of income taxes			(16.2)	(19.8)	3.2	18.1
Combined benefit fund
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Loss from discontinued operations before income taxes			$ 0.2	0.6	0.7	0.7
Patriot Coal Corporation
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net mark-to-market adjustment on actuarially determined liabilities				$ (7.9)	18.3	33.7
United Mine Workers of America
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income (loss) from discontinued operations, net of income taxes					$ 26.0	$ 36.7