Derivatives (Disclosure of Fair Value of Derivatives) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Derivative Assets	$ 55,550	$ 74,155
Derivative Liabilities	(57,259)	(69,348)
Effect of counterparty netting in derivative assets	(50,042)	(69,247)
Effect of counterparty netting in derivative liabilities	50,042	69,247
Net derivative assets as classified in the balance sheet	5,508	4,908
Net derivative liabilities as classified in the balance sheet	(7,217)	(101)
Net derivatives as classified in the balance sheet	(1,709)	4,807
Derivatives Designated as Hedging Instruments | Coal
Derivative [Line Items]
Derivative Assets	942	0
Derivative Liabilities	(2,146)	(15)
Derivatives Not Designated as Hedging Instruments
Derivative [Line Items]
Derivative Assets	54,608	74,155
Derivative Liabilities	(55,113)	(69,333)
Derivatives Not Designated as Hedging Instruments | Heating oil -- diesel purchases
Derivative [Line Items]
Derivative Assets	5,354	4,646
Derivatives Not Designated as Hedging Instruments | Coal held for trading purposes, exchange traded swaps and futures
Derivative [Line Items]
Derivative Assets	44,088	68,948
Derivative Liabilities	(45,221)	(68,740)
Derivatives Not Designated as Hedging Instruments | Coal
Derivative [Line Items]
Derivative Assets	5,139	475
Derivative Liabilities	(9,892)	(580)
Derivatives Not Designated as Hedging Instruments | Natural gas
Derivative [Line Items]
Derivative Assets	27	86
Derivative Liabilities	0	(13)
Successor
Derivative [Line Items]
Net derivative assets as classified in the balance sheet	7,339
Net derivative liabilities as classified in the balance sheet	$ (7,217)
Predecessor
Derivative [Line Items]
Net derivative assets as classified in the balance sheet		4,908
Net derivative liabilities as classified in the balance sheet		$ (101)