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Coal Trading Revenues by Type of Instrument (Details) - Coal Trading [Member] - USD ($) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Apr. 01, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Predecessor
Principal Transaction Revenue [Line Items]
Trading Gain (Loss)	$ 15.0		$ 28.9	$ 40.6
Predecessor | Commodity Swaps and Options [Member]
Principal Transaction Revenue [Line Items]
Trading Gain (Loss)	(10.2)		(66.5)	107.3
Predecessor | Physical commodity purchase / sale contracts [Member]
Principal Transaction Revenue [Line Items]
Trading Gain (Loss)	$ 25.2		$ 95.4	$ (66.7)
Successor
Principal Transaction Revenue [Line Items]
Trading Gain (Loss)		$ 33.6
Successor | Commodity Swaps and Options [Member]
Principal Transaction Revenue [Line Items]
Trading Gain (Loss)		(37.7)
Successor | Physical commodity purchase / sale contracts [Member]
Principal Transaction Revenue [Line Items]
Trading Gain (Loss)		$ 71.3

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- Definition

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+ References

No definition available.

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- Definition

Amount of gain (loss) resulting from the difference between the acquisition price and the selling price or fair value of trading assets and trading liabilities, conducted separately from customer trading activities. Includes, but is not limited to, changes in fair value of trading assets and trading liabilities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.13(e))
-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 940
-SubTopic 320
-Section 35
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=35711043&loc=d3e42454-110968

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 940
-SubTopic 320
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=35711157&loc=d3e42567-110969

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4F
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624186-113959

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