Derivatives (Net Derivatives as Reflected on the Balance Sheets) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Net derivatives as classified in the balance sheet	$ 19,942	$ 30,578
Other Current Assets [Member] | Heating Oil [Member]
Derivatives, Fair Value [Line Items]
Net derivatives as classified in the balance sheet	5,103	9,340
Coal Derivative Assets [Member] | Coal [Member]
Derivatives, Fair Value [Line Items]
Net derivatives as classified in the balance sheet	14,851	22,975
Coal Derivative Liabilities [Member] | Coal [Member]
Derivatives, Fair Value [Line Items]
Net derivatives as classified in the balance sheet	$ (12)	$ (1,737)