Derivatives (Disclosure Of Fair Value Of Derivatives) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Derivative [Line Items]
Derivative Assets	$ 42,646	$ 18,879
Derivative Liabilities	(33,292)	(31,089)
Effect of counterparty netting in derivative assets	(32,663)	(17,801)
Effect of counterparty netting in derivative liabilities	32,663	17,801
Derivative Asset	9,983	1,078
Derivative Liability	(629)	(13,288)
Net derivatives as classified in the balance sheets	9,354	(12,210)
Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	38,804	16,537
Derivative Liabilities	(32,621)	(30,284)
Not Designated as Hedging Instrument | Coal Contract
Derivative [Line Items]
Derivative Assets	20,116	10,329
Derivative Liabilities	(20,925)	(10,701)
Coal Contract | Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	3,842	2,342
Derivative Liabilities	(671)	(805)
Coal Contract | Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	18,305	5,672
Derivative Liabilities	(11,067)	(19,579)
Heating oil — diesel purchases | Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	383	532
Derivative Liabilities	(629)	0
Natural gas | Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	0	4
Derivative Liabilities	$ 0	$ (4)