Derivatives (Disclosure Of Fair Value Of Derivatives) (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Derivative [Line Items]
Derivative Assets	$ 26,039	$ 31,616
Derivative Liabilities	(20,380)	(25,870)
Effect of counterparty netting in derivative assets	(20,380)	(25,759)
Effect of counterparty netting in derivative liabilities	20,380	25,759
Derivative Asset	5,659	5,857
Derivative Liability		(111)
Net derivatives as classified in the balance sheets	5,659	5,746
Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	24,992	30,265
Derivative Liabilities	(19,529)	(24,908)
Not Designated as Hedging Instrument | Coal Contract
Derivative [Line Items]
Derivative Assets	12,910	18,467
Derivative Liabilities	(12,870)	(18,940)
Coal Contract | Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	1,047	1,351
Derivative Liabilities	(851)	(962)
Coal Contract | Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	11,554	11,662
Derivative Liabilities	(6,659)	(5,856)
Heating oil - diesel purchases | Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	528	133
Derivative Liabilities		(112)
Natural gas | Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	0	3
Derivative Liabilities	$ 0	$ 0