Derivatives (Disclosure Of Fair Value Of Derivatives) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Derivative [Line Items]
Derivative Assets	$ 50,526	$ 55,550
Derivative Liabilities	(79,406)	(57,259)
Effect of counterparty netting in derivative assets	(42,851)	(50,042)
Effect of counterparty netting in derivative liabilities	42,851	50,042
Derivative Asset	7,675	5,508
Derivative Liability	(36,555)	(7,217)
Net derivatives as classified in the balance sheets	(28,880)	(1,709)
Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	50,216	54,608
Derivative Liabilities	(71,427)	(55,113)
Not Designated as Hedging Instrument | Coal Contract
Derivative [Line Items]
Derivative Assets	38,895	44,088
Derivative Liabilities	(40,006)	(45,221)
Coal Contract | Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	310	942
Derivative Liabilities	(7,979)	(2,146)
Coal Contract | Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	5,057	5,139
Derivative Liabilities	(31,397)	(9,892)
Heating oil — diesel purchases | Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	6,240	5,354
Derivative Liabilities	0	0
Natural gas | Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	24	27
Derivative Liabilities	$ (24)	$ 0