Derivatives (Disclosure Of Fair Value Of Derivatives) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Derivative [Line Items]
Derivative Assets	$ 105,617	$ 42,263
Derivative Liabilities	(52,512)	(25,962)
Effect of counterparty netting in derivative assets	(45,422)	(18,134)
Effect of counterparty netting in derivative liabilities	45,422	18,134
Net derivative assets as classified in the balance sheet	60,195	24,129
Net derivative liabilities as classified in the balance sheet	(7,090)	(7,828)
Net derivatives as classified in the balance sheet	53,105	16,301
Heating Oil-Diesel Purchases [Member]
Derivative [Line Items]
Derivative Assets		8,997
Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative Assets	5,156	10,106
Derivative Liabilities	(1,284)
Designated as Hedging Instrument [Member] | Coal [Member]
Derivative [Line Items]
Derivative Assets	5,156	1,109
Derivative Liabilities	(1,284)
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative Assets	100,461	32,157
Derivative Liabilities	(51,228)	(25,962)
Not Designated as Hedging Instrument [Member] | Coal Held for Trading Purposes [Member]
Derivative [Line Items]
Derivative Assets	37,492	15,505
Derivative Liabilities	(38,921)	(19,927)
Not Designated as Hedging Instrument [Member] | Coal [Member]
Derivative [Line Items]
Derivative Assets	56,125	14,855
Derivative Liabilities	(12,307)	(6,035)
Not Designated as Hedging Instrument [Member] | Heating Oil-Diesel Purchases [Member]
Derivative [Line Items]
Derivative Assets	5,534
Not Designated as Hedging Instrument [Member] | Heating Oil-Fuel Surchages [Member]
Derivative [Line Items]
Derivative Assets	$ 1,310	$ 1,797