Coal Trading - Trading Revenues and Balance Sheet Offsetting (Details) - USD ($) $ in Millions		3 Months Ended			6 Months Ended	9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Apr. 01, 2017	Sep. 30, 2017	Sep. 30, 2018	Dec. 31, 2017
Successor | Coal Trading
Trading Revenues by Type of Instrument [Line Items]
Trading Revenues (Losses) by Type of Instrument		$ 22.6	$ 19.4		$ 24.6	$ 52.7
Coal trading derivative instruments and balance sheet offsetting disclosures:
Assets from coal trading activities, gross amounts of recognized assets	[1]	81.8				81.8	$ 77.1
Liabilities from coal trading activities, gross amounts of recognized liabilities	[1]	(160.4)				(160.4)	(122.0)
Assets and (liabilities) from coal trading activities, net amounts recognized before the application of variation margin	[1]	(78.6)				(78.6)	(44.9)
Gross amounts of coal trading liabilities offset against associated coal trading assets		(81.5)				(81.5)	(74.5)
Gross amounts of coal trading assets offset against associated coal trading liabilities		81.5				81.5	74.5
Net coal trading (liabilities) assets offset against associated assets (liabilities)		0.0				0.0	0.0
Variation margin held offset against assets from coal trading activities		0.0				0.0	0.0
Variation margin posted offset against liabilities from coal trading activities		69.3				69.3	35.8
Net variation margin (held) posted		69.3				69.3	35.8
Assets from coal trading activities, net		0.3				0.3	2.6
Liabilities from coal trading activities, net		(9.6)				(9.6)	(11.7)
Net assets (liabilities) from coal trading activities		(9.3)				(9.3)	(9.1)
Successor | Coal Trading Positions
Coal trading derivative instruments and balance sheet offsetting disclosures:
Liabilities from coal trading activities, gross amounts of recognized liabilities	[1]	75.2				75.2	$ 38.9
Successor | Futures, swaps and options | Coal Trading
Trading Revenues by Type of Instrument [Line Items]
Trading Revenues (Losses) by Type of Instrument		(12.1)	(17.1)		(24.4)	(44.3)
Successor | Physical purchase/sale contracts | Coal Trading
Trading Revenues by Type of Instrument [Line Items]
Trading Revenues (Losses) by Type of Instrument		$ 34.7	$ 36.5		$ 49.0	$ 97.0
Predecessor | Coal Trading
Trading Revenues by Type of Instrument [Line Items]
Trading Revenues (Losses) by Type of Instrument				$ 15.0
Predecessor | Futures, swaps and options | Coal Trading
Trading Revenues by Type of Instrument [Line Items]
Trading Revenues (Losses) by Type of Instrument				(10.2)
Predecessor | Physical purchase/sale contracts | Coal Trading
Trading Revenues by Type of Instrument [Line Items]
Trading Revenues (Losses) by Type of Instrument				$ 25.2

[1] Amounts include net liabilities of $75.2 million and $38.9 million at September 30, 2018 and December 31, 2017, respectively, representing the fair value of financial contracts used to hedge future coal sales, as further described in Note 3. “Revenue Recognition.”