Derivatives (Disclosure of Fair Value of Derivatives) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Derivative Assets	$ 31,616	$ 18,879
Derivative liabilities	(25,870)	(31,089)
Effect of counterparty netting in derivative assets	(25,759)	(17,801)
Effect of counterparty netting in derivative liabilities	25,759	17,801
Net derivative assets as classified in the balance sheet	5,857	1,078
Net derivative liabilities as classified in the balance sheet	(111)	(13,288)
Net derivatives as classified in the balance sheet	5,746	(12,210)
Derivatives Not Designated as Hedging Instruments
Derivative [Line Items]
Derivative Assets	30,265	16,537
Derivative liabilities	(24,908)	(30,284)
Derivatives Not Designated as Hedging Instruments | Coal held for trading purposes, exchange traded swaps and futures
Derivative [Line Items]
Derivative Assets	18,467	10,329
Derivative liabilities	(18,940)	(10,701)
Coal | Derivatives Designated as Hedging Instruments
Derivative [Line Items]
Derivative Assets	1,351	2,342
Derivative liabilities	(962)	(805)
Coal | Derivatives Not Designated as Hedging Instruments
Derivative [Line Items]
Derivative Assets	11,662	5,672
Derivative liabilities	(5,856)	(19,579)
Heating oil -- diesel purchases | Derivatives Not Designated as Hedging Instruments
Derivative [Line Items]
Derivative Assets	133	532
Derivative liabilities	(112)	0
Natural gas | Derivatives Not Designated as Hedging Instruments
Derivative [Line Items]
Derivative Assets	3	4
Derivative liabilities	$ 0	$ (4)