Derivatives (Disclosure Of Fair Value Of Derivatives) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Derivative [Line Items]
Derivative Assets	$ 72,842	$ 54,863
Derivative Liabilities	(46,195)	(24,285)
Effect of counterparty netting in derivative assets	(45,552)	(22,548)
Effect of counterparty netting in derivative liabilities	45,552	22,548
Net derivative assets as classified in the balance sheet	27,290	32,315
Net derivative liabilities as classified in the balance sheet	(643)	(1,737)
Net derivatives as classified in the balance sheet	26,647	30,578
Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative Assets
Derivative Liabilities
Designated as Hedging Instrument [Member] | Coal [Member]
Derivative [Line Items]
Derivative Assets	2,379	3,277
Derivative Liabilities	(252)	(10)
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative Assets	70,463	51,586
Derivative Liabilities	(45,943)	(24,275)
Not Designated as Hedging Instrument [Member] | Coal Held for Trading Purposes [Member]
Derivative [Line Items]
Derivative Assets	41,675	17,403
Derivative Liabilities	(42,234)	(16,933)
Not Designated as Hedging Instrument [Member] | Coal [Member]
Derivative [Line Items]
Derivative Assets	22,354	24,843
Derivative Liabilities	(3,709)	(7,342)
Not Designated as Hedging Instrument [Member] | Heating Oil-Diesel Purchases [Member]
Derivative [Line Items]
Derivative Assets	6,227	7,379
Derivative Liabilities	0
Not Designated as Hedging Instrument [Member] | Heating Oil-Fuel Surchages [Member]
Derivative [Line Items]
Derivative Assets	207	1,961
Derivative Liabilities