Derivative Instruments (Disclosure Of Fair Value Of Derivatives) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Derivative [Line Items]
Asset Derivative	$ 42,263	$ 51,068
Liability Derivative	(25,962)	(27,349)
Effect of counterparty netting, Asset Derivative	(18,134)	(22,402)
Effect of counterparty netting, Liability Derivative	18,134	22,402
Net derivative assets as classified in the balance sheet	24,129	28,666
Net derivative liabilities as classified in the balance sheet	(7,828)	(4,947)
Net derivatives as classified in the balance sheet	16,301	23,719
Designated As Hedging Instrument [Member]
Derivative [Line Items]
Asset Derivative	10,106	15,484
Liability Derivative		(2,350)
Designated As Hedging Instrument [Member] | Heating Oil - Diesel Purchases [Member]
Derivative [Line Items]
Asset Derivative	8,997	13,475
Liability Derivative
Designated As Hedging Instrument [Member] | Coal [Member]
Derivative [Line Items]
Asset Derivative	1,109	2,009
Liability Derivative		(2,350)
Not Designated As Hedging Instrument [Member]
Derivative [Line Items]
Asset Derivative	32,157	35,584
Liability Derivative	(25,962)	(24,999)
Not Designated As Hedging Instrument [Member] | Heating Oil - Fuel Surcharges [Member]
Derivative [Line Items]
Asset Derivative	1,797
Liability Derivative
Not Designated As Hedging Instrument [Member] | Coal Held For Trading Purposes [Member]
Derivative [Line Items]
Asset Derivative	15,505	34,445
Liability Derivative	(19,927)	(24,087)
Not Designated As Hedging Instrument [Member] | Coal [Member]
Derivative [Line Items]
Asset Derivative	14,855	1,139
Liability Derivative	$ (6,035)	$ (912)