Derivative and Hedging Instruments (Details-3) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Offsetting Liabilities [Line Items]
Collateral posted against derivative positions	$ (100)	$ 0
Commodity Derivative Contracts [Member]
Offsetting Liabilities [Line Items]
Gross amounts	793	218
Gross amounts offset in balance sheet	(654)	(218)
Collateral posted against derivative positions	(139)	0
Net amount presented in the balance sheet	0	0
PPA [Member]
Offsetting Liabilities [Line Items]
Gross amounts	42,170	50,590
Gross amounts offset in balance sheet	0	0
Collateral posted against derivative positions	0	0
Net amount presented in the balance sheet	$ 42,170	$ 50,590