Derivatives (Net Derivatives As Reflected On The Balance Sheets) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Net derivatives as classified in the balance sheet	$ 34,311	$ 30,578
Other Current Assets [Member] | Heating Oil [Member]
Derivatives, Fair Value [Line Items]
Net derivatives as classified in the balance sheet	3,592	9,340
Coal Derivative Assets [Member] | Coal [Member]
Derivatives, Fair Value [Line Items]
Net derivatives as classified in the balance sheet	31,261	22,975
Coal Derivative Liabilities [Member] | Coal [Member]
Derivatives, Fair Value [Line Items]
Net derivatives as classified in the balance sheet	$ (542)	$ (1,737)