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Coal Trading - Trading Revenues and Balance Sheet Offsetting (Details) - Coal Trading - USD ($) $ in Millions		3 Months Ended			6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Apr. 01, 2017	Jun. 30, 2018	Dec. 31, 2017
Successor
Trading Revenues by Type of Instrument [Line Items]
Trading Revenues by Type of Instrument		$ 10.0	$ 5.2		$ 30.1
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]	(137.1)			(137.1)	(122.0)
Assets and (liabilities) from coal trading activities, net amounts recognized before the application of variation margin	[1]	(55.3)			(55.3)	(44.9)
Gross amounts of coal trading liabilities offset against associated coal trading assets		(74.6)			(74.6)	(74.5)
Gross amounts of coal trading assets offset against associated coal trading liabilities		74.6			74.6	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		47.5			47.5	35.8
Net variation margin (held) posted		47.5			47.5	35.8
Assets from coal trading activities, net		7.2			7.2	2.6
Liabilities from coal trading activities, net		(15.0)			(15.0)	(11.7)
Net assets (liabilities) from coal trading activities		(7.8)			(7.8)	$ (9.1)
Successor | Futures, swaps and options
Trading Revenues by Type of Instrument [Line Items]
Trading Revenues by Type of Instrument		(15.3)	(7.3)		(32.2)
Successor | Physical purchase/sale contracts
Trading Revenues by Type of Instrument [Line Items]
Trading Revenues by Type of Instrument		$ 25.3	$ 12.5		$ 62.3
Predecessor
Trading Revenues by Type of Instrument [Line Items]
Trading Revenues by Type of Instrument				$ 15.0
Predecessor | Futures, swaps and options
Trading Revenues by Type of Instrument [Line Items]
Trading Revenues by Type of Instrument				(10.2)
Predecessor | Physical purchase/sale contracts
Trading Revenues by Type of Instrument [Line Items]
Trading Revenues by Type of Instrument				$ 25.2
[1] Amounts include net liabilities of $48.4 million and $38.9 million at June 30, 2018 and December 31, 2017, respectively, representing the fair value of financial contracts related to hedge activities intended to secure pricing under certain coal sales contracts, as further described in Note 3. “Revenue Recognition.”

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