Derivatives (Disclosure Of Fair Value Of Derivatives) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Derivative [Line Items]
Derivative Assets	$ 91,190	$ 42,263
Derivative Liabilities	(40,156)	(25,962)
Effect of counterparty netting in derivative assets	(36,658)	(18,134)
Effect of counterparty netting in derivative liabilities	36,658	18,134
Net derivative assets as classified in the balance sheet	54,532	24,129
Net derivative liabilities as classified in the balance sheet	(3,498)	(7,828)
Net derivatives as classified in the balance sheet	51,034	16,301
Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative Assets	4,509	10,106
Derivative Liabilities	(1,679)
Designated as Hedging Instrument [Member] | Coal [Member]
Derivative [Line Items]
Derivative Assets	4,509	1,109
Derivative Liabilities	(1,679)
Designated as Hedging Instrument [Member] | Heating Oil-Diesel Purchases [Member]
Derivative [Line Items]
Derivative Assets		8,997
Derivative Liabilities
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative Assets	86,681	32,157
Derivative Liabilities	(38,477)	(25,962)
Not Designated as Hedging Instrument [Member] | Coal Held for Trading Purposes [Member]
Derivative [Line Items]
Derivative Assets	27,190	15,505
Derivative Liabilities	(25,171)	(19,927)
Not Designated as Hedging Instrument [Member] | Coal [Member]
Derivative [Line Items]
Derivative Assets	45,739	14,855
Derivative Liabilities	(13,249)	(6,035)
Not Designated as Hedging Instrument [Member] | Heating Oil-Diesel Purchases [Member]
Derivative [Line Items]
Derivative Assets	10,913
Derivative Liabilities	(57)
Not Designated as Hedging Instrument [Member] | Heating Oil-Fuel Surchages [Member]
Derivative [Line Items]
Derivative Assets	2,839	1,797
Derivative Liabilities