Derivatives (Disclosure Of Fair Value Of Derivatives) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Derivative [Line Items]
Derivative Assets	$ 31,364	$ 31,616
Derivative Liabilities	(26,206)	(25,870)
Effect of counterparty netting in derivative assets	(26,205)	(25,759)
Effect of counterparty netting in derivative liabilities	26,205	25,759
Derivative Asset	5,159	5,857
Derivative Liability	(1)	(111)
Net derivatives as classified in the balance sheets	5,158	5,746
Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	30,232	30,265
Derivative Liabilities	(25,367)	(24,908)
Not Designated as Hedging Instrument | Coal Contract
Derivative [Line Items]
Derivative Assets	19,565	18,467
Derivative Liabilities	(19,667)	(18,940)
Coal Contract | Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	1,132	1,351
Derivative Liabilities	(839)	(962)
Coal Contract | Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	10,466	11,662
Derivative Liabilities	(5,699)	(5,856)
Heating oil - diesel purchases | Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	201	133
Derivative Liabilities	(1)	(112)
Natural gas | Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	0	3
Derivative Liabilities	$ 0	$ 0