Derivatives (Net Derivatives as Reflected on the Balance Sheets) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Net derivatives as classified in the balance sheet	$ (1,709)	$ 4,807
Other current assets | Heating oil
Derivatives, Fair Value [Line Items]
Net derivatives as classified in the balance sheet	5,354	4,646
Other current assets | Coal
Derivatives, Fair Value [Line Items]
Net derivatives as classified in the balance sheet	154	262
Accrued expenses and other current liabilities | Coal
Derivatives, Fair Value [Line Items]
Net derivatives as classified in the balance sheet	$ (7,217)	$ (101)