Derivative Instruments (Net Derivatives As Reflected On The Balance Sheets) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Derivative [Line Items]
Derivative assets	$ 42,263	$ 51,068
Net derivatives as reflected on the balance sheet	16,301	23,719
Coal [Member]
Derivative [Line Items]
Coal derivative assets	13,335	15,191
Coal derivative liabilities	(7,828)	(4,947)
Other Current Assets [Member] | Heating Oil [Member]
Derivative [Line Items]
Derivative assets	$ 10,794	$ 13,475