Derivative and Hedging Instruments (Details-3) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Offsetting Liabilities [Line Items]
Collateral posted against derivative positions	$ (800)	$ (2,200)
Commodity Derivative Contracts [Member]
Offsetting Liabilities [Line Items]
Gross amounts	1,258	2,433
Gross amounts offset in balance sheet	(431)	(223)
Collateral posted against derivative positions	(827)	(2,179)
Net amount presented in balance sheet	0	31
Purchased Power Agreement [Member]
Offsetting Liabilities [Line Items]
Gross amounts	56,950	53,430
Gross amounts offset in balance sheet	0	0
Collateral posted against derivative positions	0	0
Net amount presented in balance sheet	$ 56,950	$ 53,430