Derivatives (Net Derivatives as Reflected on the Balance Sheets) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Derivatives, Fair Value [Line Items]
Net derivatives as classified in the balance sheet	$ 5,746	$ (12,210)
Other current assets | Heating oil
Derivatives, Fair Value [Line Items]
Net derivatives as classified in the balance sheet	133	532
Other current assets | Coal
Derivatives, Fair Value [Line Items]
Net derivatives as classified in the balance sheet	5,724	546
Accrued expenses and other current liabilities | Coal
Derivatives, Fair Value [Line Items]
Net derivatives as classified in the balance sheet	$ (111)	$ (13,288)