Derivatives (Disclosure Of Fair Value Of Derivatives) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Derivative [Line Items]
Derivative Assets	$ 45,861	$ 74,155
Derivative Liabilities	(46,126)	(69,348)
Effect of counterparty netting in derivative assets	(43,830)	(69,247)
Effect of counterparty netting in derivative liabilities	43,830	69,247
Derivative Asset	2,031	4,908
Derivative Liability	(2,296)	(101)
Net derivatives as classified in the balance sheet	(265)	4,807
Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	45,858	74,155
Derivative Liabilities	(46,126)	(69,333)
Not Designated as Hedging Instrument | Coal Contract
Derivative [Line Items]
Derivative Assets	42,203	68,948
Derivative Liabilities	(44,211)	(68,740)
Coal Contract | Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	3	0
Derivative Liabilities	0	(15)
Coal Contract | Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	1,624	475
Derivative Liabilities	(1,915)	(580)
Heating oil — diesel purchases | Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	2,006	4,646
Derivative Liabilities	0	0
Natural gas — unrealized | Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	25	86
Derivative Liabilities	$ 0	$ (13)