Derivatives (Disclosure Of Fair Value Of Derivatives) (Details) (USD $) In Thousands	Sep. 30, 2011	Dec. 31, 2010
Derivative Assets	$ 26,936	$ 51,068
Derivative Liabilities	(18,611)	(27,349)
Effect of counterparty netting in derivative assets	(12,787)	(22,402)
Effect of counterparty netting in derivative liabilities	12,787	22,402
Net derivative assets as classified in the balance sheet	14,149	28,666
Net derivative liabilities as classified in the balance sheet	(5,824)	(4,947)
Net derivatives as classified in the balance sheet	8,325	23,719
Coal [Member] | Designated as Hedging Instrument [Member]
Derivative Assets	2,708	2,009
Derivative Liabilities	(1,270)	(2,350)
Heating Oil-diesel purchases [Member] | Designated as Hedging Instrument [Member]
Derivative Assets	9,352	13,475
Derivative Liabilities
Designated as Hedging Instrument [Member]
Derivative Assets	12,060	15,484
Derivative Liabilities	(1,270)	(2,350)
Coal [Member] | Not Designated as Hedging Instrument [Member]
Derivative Assets	1,812	1,139
Derivative Liabilities	(6,976)	(912)
Heating oil-fuel surchages [Member] | Not Designated as Hedging Instrument [Member]
Derivative Assets	2,202
Not Designated as Hedging Instrument [Member]
Derivative Assets	14,876	35,584
Derivative Liabilities	(17,341)	(24,999)
Not Designated as Hedging Instrument [Member] | Coal Held for Trading Purposes [Member]
Derivative Assets	10,862	34,445
Derivative Liabilities	$ (10,365)	$ (24,087)