Derivatives (Disclosure Of Fair Value Of Derivatives) (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Liability	$ (120,357)	$ (98,686)
Derivative Liability, Fair Value, Gross Asset	120,357	98,686
Derivative Asset	14,628	13,557
Derivative Liability	(536)	(766)
Net derivatives as classified in the balance sheet	14,092	12,791
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Price Risk Derivative Instruments Not Designated as Hedging Instruments Asset, at Fair Value	133,377	105,708
Price Risk Derivative Instruments Not Designated as Hedging Instruments Liability, at Fair Value	(120,893)	(96,960)
Not Designated as Hedging Instrument [Member] | Coal Contract [Member]
Derivative [Line Items]
Derivative Assets	121,312	96,898
Derivative Liabilities	(114,636)	(93,272)
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	600	300
Price Risk Derivative Instruments Not Designated as Hedging Instruments Liability, at Fair Value	0	0
Coal Contract [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative Assets	1,608	6,535
Derivative Liabilities	0	(2,492)
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	9,965	8,510
Price Risk Derivative Instruments Not Designated as Hedging Instruments Liability, at Fair Value	(6,257)	(3,688)
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	1,215	0
Price Risk Derivative Instruments Not Designated as Hedging Instruments Liability, at Fair Value	0	0
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	285
Price Risk Derivative Instruments Not Designated as Hedging Instruments Liability, at Fair Value	0
Price Risk Derivative [Member]
Derivative [Line Items]
Derivative Assets	134,985	112,243
Derivative Liabilities	$ (120,893)	$ (99,452)