Derivatives (Disclosure Of Fair Value Of Derivatives) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Derivative [Line Items]
Derivative Assets	$ 54,863	$ 42,263
Derivative Liabilities	(24,285)	(25,962)
Effect of counterparty netting in derivative assets	(22,548)	(18,134)
Effect of counterparty netting in derivative liabilities	22,548	18,134
Net derivative assets as classified in the balance sheet	32,315	24,129
Net derivative liabilities as classified in the balance sheet	(1,737)	(7,828)
Net derivatives as classified in the balance sheet	30,578	16,301
Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative Assets	3,277	10,106
Derivative Liabilities	(10)
Designated as Hedging Instrument [Member] | Coal [Member]
Derivative [Line Items]
Derivative Assets	3,277	1,109
Derivative Liabilities	(10)
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	51,586	32,157
Derivative Liabilities	(24,275)	(25,962)
Not Designated as Hedging Instrument [Member] | Coal Held for Trading Purposes [Member]
Derivative [Line Items]
Derivative Assets	17,403	15,505
Derivative Liabilities	(16,933)	(19,927)
Not Designated as Hedging Instrument [Member] | Coal [Member]
Derivative [Line Items]
Derivative Assets	24,843	14,855
Derivative Liabilities	(7,342)	(6,035)
Not Designated as Hedging Instrument [Member] | Heating Oil-Diesel Purchases [Member]
Derivative [Line Items]
Derivative Assets	7,379
Derivative Liabilities	0
Not Designated as Hedging Instrument [Member] | Heating Oil-Fuel Surchages [Member]
Derivative [Line Items]
Derivative Assets	1,961	1,797
Derivative Liabilities