Derivatives (Disclosure Of Fair Value Of Derivatives) (Details) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Derivative Assets	$ 41,506	$ 51,068
Derivative Liabilities	(16,416)	(27,349)
Effect of counterparty netting in derivative assets	(10,625)	(22,402)
Effect of counterparty netting in derivative liabilities	10,625	22,402
Net derivative assets as classified in the balance sheet	30,881	28,666
Net derivative liabilities as classified in the balance sheet	(5,791)	(4,947)
Net derivatives as classified in the balance sheet	25,090	23,719
Coal [Member] | Designated as Hedging Instrument [Member]
Derivative Assets	3,273	2,009
Derivative Liabilities	(1,924)	(2,350)
Designated as Hedging Instrument [Member]
Derivative Assets	21,374	15,484
Derivative Liabilities	(1,924)	(2,350)
Designated as Hedging Instrument [Member] | Heating Oil [Member]
Derivative Assets	18,101	13,475
Derivative Liabilities
Coal [Member] | Not Designated as Hedging Instrument [Member]
Derivative Assets	1,352	1,139
Derivative Liabilities	(4,704)	(912)
Not Designated as Hedging Instrument [Member]
Derivative Assets	20,132	35,584
Derivative Liabilities	(14,492)	(24,999)
Not Designated as Hedging Instrument [Member] | Coal Held for Trading Purposes [Member]
Derivative Assets	18,780	34,445
Derivative Liabilities	$ (9,788)	$ (24,087)