Coal Trading - Fair Value (Details) - Successor - USD ($) $ in Millions		Jun. 30, 2018	Dec. 31, 2017
Coal Trading Positions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability, Fair Value, Gross Liability	[1]	$ (48.4)	$ (38.9)
Coal Trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability, Fair Value, Gross Liability	[1]	137.1	122.0
Fair Value, Measurements, Recurring | Coal Trading
Fair value coal trading net assets (liabilities) measured on recurring basis:
Futures, swaps and options		(0.3)	(7.2)
Physical purchase/sale contracts		(7.5)	(1.9)
Total net financial assets (liabilities)		(7.8)	(9.1)
Level 1 | Fair Value, Measurements, Recurring | Coal Trading
Fair value coal trading net assets (liabilities) measured on recurring basis:
Futures, swaps and options		0.0	(3.0)
Physical purchase/sale contracts		0.0	0.0
Total net financial assets (liabilities)		0.0	(3.0)
Level 2 | Fair Value, Measurements, Recurring | Coal Trading
Fair value coal trading net assets (liabilities) measured on recurring basis:
Futures, swaps and options		(0.3)	(4.2)
Physical purchase/sale contracts		(7.5)	(1.9)
Total net financial assets (liabilities)		(7.8)	(6.1)
Level 3 | Fair Value, Measurements, Recurring | Coal Trading
Fair value coal trading net assets (liabilities) measured on recurring basis:
Futures, swaps and options		0.0	0.0
Physical purchase/sale contracts		0.0	0.0
Total net financial assets (liabilities)		$ 0.0	$ 0.0

[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.”