Derivatives (Disclosure Of Fair Value Of Derivatives) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Liability	$ (64,965)	$ (107,028)
Derivative Liability, Fair Value, Gross Asset	64,965	107,028
Derivative Asset	6,153	9,052
Derivative Liability	(355)	0
Net derivatives as classified in the balance sheet	5,798	9,052
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Price Risk Derivative Instruments Not Designated as Hedging Instruments Asset, at Fair Value	71,118	116,076
Price Risk Derivative Instruments Not Designated as Hedging Instruments Liability, at Fair Value	(65,306)	(107,008)
Not Designated as Hedging Instrument [Member] | Coal Contract [Member]
Derivative [Line Items]
Derivative Assets	66,156	110,653
Derivative Liabilities	(64,730)	(104,814)
Coal Contract [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative Assets	0	4
Derivative Liabilities	(14)	(20)
Coal Contract [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Price Risk Derivative Instruments Not Designated as Hedging Instruments Asset, at Fair Value	879	3,912
Price Risk Derivative Instruments Not Designated as Hedging Instruments Liability, at Fair Value	(576)	(1,947)
Heating Oil-Diesel Purchases [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Price Risk Derivative Instruments Not Designated as Hedging Instruments Asset, at Fair Value	3,964	1,017
Price Risk Derivative Instruments Not Designated as Hedging Instruments Liability, at Fair Value	0	0
Natural Gas [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Price Risk Derivative Instruments Not Designated as Hedging Instruments Asset, at Fair Value	110	494
Price Risk Derivative Instruments Not Designated as Hedging Instruments Liability, at Fair Value	0	(247)
Foreign Currency [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Price Risk Derivative Instruments Not Designated as Hedging Instruments Asset, at Fair Value	9	0
Price Risk Derivative Instruments Not Designated as Hedging Instruments Liability, at Fair Value	0	0
Price Risk Derivative [Member]
Derivative [Line Items]
Derivative Assets	71,118	116,080
Derivative Liabilities	$ (65,320)	$ (107,028)