Derivative and Hedging Instruments (Details-3) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Offsetting Liabilities [Line Items]
Collateral posted against derivative positions	$ (100)	$ (100)
Commodity Derivative Contracts [Member]
Offsetting Liabilities [Line Items]
Gross amounts	483	793
Gross amounts offset in balance sheet	(390)	(654)
Collateral posted against derivative positions	(93)	(139)
Net amount presented in balance sheet	0	0
Purchased Power Agreement [Member]
Offsetting Liabilities [Line Items]
Gross amounts	41,290	42,170
Gross amounts offset in balance sheet	0	0
Collateral posted against derivative positions	0	0
Net amount presented in balance sheet	$ 41,290	$ 42,170