Derivatives (Disclosure Of Fair Value Of Derivatives) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Derivative Assets	$ 62,857	$ 18,879
Derivative Liabilities	(57,490)	(31,089)
Effect of counterparty netting in derivative assets	(57,490)	(17,801)
Effect of counterparty netting in derivative liabilities	57,490	17,801
Derivative Asset	5,367	1,078
Derivative Liability	0	(13,288)
Net derivatives as classified in the balance sheets	5,367	(12,210)
Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	57,396	16,537
Derivative Liabilities	(57,193)	(30,284)
Not Designated as Hedging Instrument | Coal Contract
Derivative [Line Items]
Derivative Assets	33,091	10,329
Derivative Liabilities	(33,932)	(10,701)
Coal Contract | Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	5,461	2,342
Derivative Liabilities	(297)	(805)
Coal Contract | Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	22,552	5,672
Derivative Liabilities	(23,033)	(19,579)
Heating oil — diesel purchases | Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	1,734	532
Derivative Liabilities	(228)	0
Natural gas | Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	19	4
Derivative Liabilities	$ 0	$ (4)