Fair Value of Assets and Liabilities from Coal Trading Activities (Details) - Coal Trading [Member] - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Predecessor
Fair Value of Assets and Liabilities from Coal Trading Activities [Line Items]
Derivative Asset, Fair Value, Gross Asset		$ 191.2
Derivative Liability, Fair Value, Gross Liability		249.1
Derivative, Fair Value, Net		(57.9)
Derivative Asset, Fair Value, Gross Liability		(190.5)
Derivative Liability, Fair Value, Gross Asset		190.5
Derivative Fair Value Gross Basis Reclassifications Net		0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral		0.0
Derivative Liability, Fair Value, Amount Offset Against Collateral		(57.4)
Derivative, Fair Value, Amount Offset Against Collateral, Net		57.4
Derivative Asset		0.7
Derivative Liability		1.2
Derivative Assets (Liabilities), at Fair Value, Net		$ (0.5)
Successor
Fair Value of Assets and Liabilities from Coal Trading Activities [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 77.1
Derivative Liability, Fair Value, Gross Liability	122.0
Derivative, Fair Value, Net	(44.9)
Derivative Asset, Fair Value, Gross Liability	(74.5)
Derivative Liability, Fair Value, Gross Asset	74.5
Derivative Fair Value Gross Basis Reclassifications Net	0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral	0.0
Derivative Liability, Fair Value, Amount Offset Against Collateral	(35.8)
Derivative, Fair Value, Amount Offset Against Collateral, Net	35.8
Derivative Asset	2.6
Derivative Liability	11.7
Derivative Assets (Liabilities), at Fair Value, Net	(9.1)
Cash Flow Hedging [Member]
Fair Value of Assets and Liabilities from Coal Trading Activities [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net	$ 0.0