Summary Quarterly Financial Information Summary of Quarterly Financial Information Textuals (Details) - USD ($) $ in Millions				3 Months Ended									9 Months Ended	12 Months Ended
	Nov. 28, 2017	Nov. 27, 2017	Apr. 01, 2017	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Apr. 01, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Predecessor
Quarterly Financial Information [Line Items]
Mark-to-Market Adjustment on Pension and Postretirement Plans, Net							$ 0.0							$ 0.0	$ 0.0
Payments of Debt Restructuring Costs											$ 10.3	$ 26.4
Asset impairment							30.5	$ 30.5	$ 230.7			17.2		247.9	1,277.8
Gain (Loss) on Disposition of Assets							22.8				13.7			23.2	45.0
(Income) loss from equity affiliates							(15.0)	(15.0)	(28.8)					(16.2)	15.9
Interest Expense							32.9					$ 126.2		298.6	465.4
Reorganization Items			$ 585.8				627.2	$ 41.4	33.9	$ 29.7	95.4			159.0	0.0
Loss on early debt extinguishment							0.0		29.5					(29.5)	(67.8)
Income (Loss) from discontinued operations, net of tax, including portion attributable to noncontrolling interest							(16.2)		$ 13.1	$ 38.1				(57.6)	(175.0)
Predecessor | Nonstrategic Australian mining tenement [Member]
Quarterly Financial Information [Line Items]
Gain (Loss) on Disposition of Assets											2.8
Predecessor | Prarie State Energy Campus [Member]
Quarterly Financial Information [Line Items]
Gain (Loss) on Disposition of Assets											$ 6.2
Predecessor | Burton Mine [Member]
Quarterly Financial Information [Line Items]
Gain (Loss) on Sale of Properties							0.0							0.0	0.0
Predecessor | Springfield Reclamation Liability [Member]
Quarterly Financial Information [Line Items]
Gain (Loss) on Sale of Properties							$ 0.0							$ 0.0	$ 0.0
Successor
Quarterly Financial Information [Line Items]
Mark-to-Market Adjustment on Pension and Postretirement Plans, Net				$ 45.2									$ (45.2)
Payments of Debt Restructuring Costs				6.6
Asset impairment													0.0
Gain (Loss) on Disposition of Assets				83.1									84.0
(Income) loss from equity affiliates				(22.8)	$ (10.5)	$ (15.7)							(49.0)
Interest Expense					42.5	$ 41.4							119.7
Reorganization Items													0.0
Loss on early debt extinguishment				8.0	$ 12.9								(20.9)
Income (Loss) from discontinued operations, net of tax, including portion attributable to noncontrolling interest													(19.8)
Successor | Burton Mine [Member]
Quarterly Financial Information [Line Items]
Gain (Loss) on Disposition of Assets		$ 52.2
Gain (Loss) on Sale of Properties				52.2									(52.2)
Successor | Springfield Reclamation Liability [Member]
Quarterly Financial Information [Line Items]
Gain (Loss) on Disposition of Assets	$ 31.2
Gain (Loss) on Sale of Properties				$ 31.2									$ (31.2)