Derivatives (Net Derivatives as Reflected on the Balance Sheets) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Net derivatives as classified in the balance sheet	$ (12,210)	$ (1,709)
Other current assets | Heating oil
Derivatives, Fair Value [Line Items]
Net derivatives as classified in the balance sheet	532	5,354
Other current assets | Coal
Derivatives, Fair Value [Line Items]
Net derivatives as classified in the balance sheet	546	154
Accrued expenses and other current liabilities | Coal
Derivatives, Fair Value [Line Items]
Net derivatives as classified in the balance sheet	$ (13,288)	$ (7,217)