Derivatives (Disclosure Of Fair Value Of Derivatives) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Derivative [Line Items]
Derivative Assets	$ 3,245	$ 31,616
Derivative Liabilities	(2,399)	(25,870)
Effect of counterparty netting in derivative assets	(2,392)	(25,759)
Effect of counterparty netting in derivative liabilities	2,392	25,759
Derivative Asset	853	5,857
Derivative Liability	(7)	(111)
Net derivatives as classified in the balance sheets	846	5,746
Other current assets
Derivative [Line Items]
Effect of counterparty netting in derivative assets		(4,400)
Other Current Liabilities
Derivative [Line Items]
Effect of counterparty netting in derivative liabilities	1,400
Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	3,245	30,265
Derivative Liabilities	(2,399)	(24,908)
Not Designated as Hedging Instrument | Coal held for trading purposes, exchange traded swaps and futures
Derivative [Line Items]
Derivative Assets	1,914	18,467
Derivative Liabilities	(1,595)	(18,940)
Coal | Accrued expenses and other current liabilities
Derivative [Line Items]
Net derivatives as classified in the balance sheets	(7)	(111)
Coal | Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets		1,351
Derivative Liabilities	0	(962)
Coal | Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	1,094	11,662
Derivative Liabilities	(804)	(5,856)
Heating oil - diesel purchases | Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	237	133
Derivative Liabilities	0	(112)
Natural gas | Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets		3
Derivative Liabilities	$ 0	$ 0