Derivatives (Net Derivatives as Reflected on the Balance Sheets) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Derivatives, Fair Value [Line Items]
Net derivatives as classified in the balance sheet	$ 12,791	$ 19,942
Other Current Assets [Member] | Heating Oil [Member]
Derivatives, Fair Value [Line Items]
Net derivatives as classified in the balance sheet	300	5,103
Coal Derivative Assets [Member] | Coal [Member]
Derivatives, Fair Value [Line Items]
Net derivatives as classified in the balance sheet	13,257	14,851
Coal Derivative Liabilities [Member] | Coal [Member]
Derivatives, Fair Value [Line Items]
Net derivatives as classified in the balance sheet	$ (766)	$ (12)