Derivatives (Net Derivatives As Reflected On The Balance Sheets) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Derivative assets	$ 68,398	$ 42,263
Net derivatives as reflected on the balance sheet	28,650	16,301
Derivative Liabilities	9,100	7,828
Coal [Member]
Derivative assets	22,043	13,335
Derivative Liabilities	(9,100)	(7,828)
Other Current Assets [Member] | Heating Oil [Member]
Derivative assets	$ 15,707	$ 10,794