Derivatives (Disclosure Of Fair Value Of Derivatives) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Derivative [Line Items]
Derivative Assets	$ 10,422	$ 31,616
Derivative Liabilities	(7,810)	(25,870)
Effect of counterparty netting in derivative assets	(7,757)	(25,759)
Effect of counterparty netting in derivative liabilities	7,757	25,759
Derivative Asset	2,665	5,857
Derivative Liability	(53)	(111)
Net derivatives as classified in the balance sheets	2,612	5,746
Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	9,969	30,265
Derivative Liabilities	(7,455)	(24,908)
Not Designated as Hedging Instrument | Coal Contract
Derivative [Line Items]
Derivative Assets	5,128	18,467
Derivative Liabilities	(4,965)	(18,940)
Coal Contract | Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	453	1,351
Derivative Liabilities	(355)	(962)
Coal Contract | Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	4,736	11,662
Derivative Liabilities	(2,490)	(5,856)
Heating oil - diesel purchases | Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	105	133
Derivative Liabilities		(112)
Natural gas | Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	0	3
Derivative Liabilities	$ 0	$ 0