Derivatives (Disclosure Of Fair Value Of Derivatives) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Derivative Assets	$ 32,762	$ 55,550
Derivative Liabilities	(25,985)	(57,259)
Effect of counterparty netting in derivative assets	(25,199)	(50,042)
Effect of counterparty netting in derivative liabilities	25,199	50,042
Derivative Asset	7,563	5,508
Derivative Liability	(786)	(7,217)
Net derivatives as classified in the balance sheet	6,777	(1,709)
Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	27,966	54,608
Derivative Liabilities	(25,101)	(55,113)
Not Designated as Hedging Instrument | Coal Contract
Derivative [Line Items]
Derivative Assets	19,899	44,088
Derivative Liabilities	(20,082)	(45,221)
Coal Contract | Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	4,796	942
Derivative Liabilities	(884)	(2,146)
Coal Contract | Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	3,140	5,139
Derivative Liabilities	(5,019)	(9,892)
Heating oil — diesel purchases | Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	4,927	5,354
Derivative Liabilities	0	0
Natural gas — unrealized | Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	0	27
Derivative Liabilities	$ 0	$ 0