Derivatives (Disclosure Of Fair Value Of Derivatives) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Derivative Assets	$ 68,398	$ 42,263
Derivative Liabilities	(39,748)	(25,962)
Effect of counterparty netting in derivative assets	(30,648)	(18,134)
Effect of counterparty netting in derivative liabilities	30,648	18,134
Net derivative assets as classified in the balance sheet	37,750	24,129
Net derivative liabilities as classified in the balance sheet	(9,100)	(7,828)
Net derivatives as classified in the balance sheet	28,650	16,301
Coal [Member]
Derivative Assets	22,043	13,335
Net derivative liabilities as classified in the balance sheet	9,100	7,828
Designated as Hedging Instrument [Member]
Derivative Assets	3,256	10,106
Derivative Liabilities	(218)
Designated as Hedging Instrument [Member] | Coal [Member]
Derivative Assets	3,256	1,109
Derivative Liabilities	(218)
Designated as Hedging Instrument [Member] | Heating Oil-Diesel Purchases [Member]
Derivative Assets		8,997
Not Designated as Hedging Instrument [Member]
Derivative Assets	65,142	32,157
Derivative Liabilities	(39,530)	(25,962)
Not Designated as Hedging Instrument [Member] | Coal Held for Trading Purposes [Member]
Derivative Assets	26,945	15,505
Derivative Liabilities	(33,411)	(19,927)
Not Designated as Hedging Instrument [Member] | Coal [Member]
Derivative Assets	22,491	14,855
Derivative Liabilities	(6,119)	(6,035)
Not Designated as Hedging Instrument [Member] | Heating Oil-Diesel Purchases [Member]
Derivative Assets	12,869
Not Designated as Hedging Instrument [Member] | Heating Oil-Fuel Surchages [Member]
Derivative Assets	$ 2,837	$ 1,797