Derivatives (Disclosure Of Fair Value Of Derivatives) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Derivative [Line Items]
Derivative Assets	$ 42,661	$ 74,155
Derivative Liabilities	(43,374)	(69,348)
Effect of counterparty netting in derivative assets	(41,947)	(69,247)
Effect of counterparty netting in derivative liabilities	41,947	69,247
Derivative asset excluding interest rate swaps	714
Derivative Asset	963	4,908
Derivative Liability	(1,427)	(101)
Net derivatives as classified in the balance sheet	(713)	4,807
Designated as Hedging Instrument | Coal Contract
Derivative [Line Items]
Derivative Assets	17	0
Derivative Liabilities	0	(15)
Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	42,644	74,155
Derivative Liabilities	(43,374)	(69,333)
Not Designated as Hedging Instrument | Coal Contract
Derivative [Line Items]
Derivative Assets	41,813	68,948
Derivative Liabilities	(43,157)	(68,740)
Not Designated as Hedging Instrument | Coal Contract
Derivative [Line Items]
Derivative Assets	67	475
Derivative Liabilities	(146)	(580)
Not Designated as Hedging Instrument | Heating oil — diesel purchases
Derivative [Line Items]
Derivative Assets	661	4,646
Derivative Liabilities	0	0
Not Designated as Hedging Instrument | Natural gas — unrealized
Derivative [Line Items]
Derivative Assets	103	86
Derivative Liabilities	$ (71)	$ (13)