Derivative and Hedging Instruments (Details-1) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($) Dth MWh MW	Dec. 31, 2014 USD ($) Dth MWh MW	Dec. 31, 2013 USD ($)
Gross Notional Volume of Open Derivatives
Derivative, right to reclaim collateral (receivable)	$ 1,000	$ 2,200
Other Current Assets [Member]
Derivative Fair Values [Abstract]
Derivative fair value, net	1,208	1,001	$ 411
Commodity Contracts and Financial Transimission Rights [Member]
Derivative Fair Values [Abstract]
Derivative fair value, net	$ (800)	$ (1,600)
Commodity Derivative Contracts [Member]
Gross Notional Volume of Open Derivatives
Notional amount, energy measure (in MWh) | MWh	355,580	448,000
Notional amount, decatherm measure (in Dth) | Dth	5,037,500	4,405,000
Derivative Fair Values [Abstract]
Asset Derivatives, fair value, gross basis	$ 290	$ 223
Liability Derivatives, fair value, gross basis	1,336	2,433
Commodity Derivative Contracts [Member] | Other Current Assets [Member]
Derivative Fair Values [Abstract]
Asset Derivatives, fair value, gross basis	146	130
Commodity Derivative Contracts [Member] | Other Deferred Charges [Member]
Derivative Fair Values [Abstract]
Asset Derivatives, fair value, gross basis	144	93
Commodity Derivative Contracts [Member] | Derivative Liability (Current) [Member]
Derivative Fair Values [Abstract]
Liability Derivatives, fair value, gross basis	1,266	2,262
Commodity Derivative Contracts [Member] | Derivative Liability (Long-term) [Member]
Derivative Fair Values [Abstract]
Liability Derivatives, fair value, gross basis	$ 70	$ 171
Energy Related Commodity Contract [Member] | Cash Flow Hedging [Member]
Derivatives Fair Value [Line Items]
Maximum term of derivative hedging contract	4 years
Financial Transmission Rights [Member]
Gross Notional Volume of Open Derivatives
Notional amount, power measure (in MW) | MW	2,000	1,854
Derivative Fair Values [Abstract]
Asset Derivatives, fair value, gross basis	$ 234	$ 642
Financial Transmission Rights [Member] | Other Current Assets [Member]
Derivative Fair Values [Abstract]
Asset Derivatives, fair value, gross basis	234	642
Financial Transmission Rights [Member] | Derivative Liability (Current) [Member]
Derivative Fair Values [Abstract]
Liability Derivatives, fair value, gross basis	0	0
PPA [Member]
Derivative Fair Values [Abstract]
Derivative fair value, net	(53,300)	(53,400)
Liability Derivatives, fair value, gross basis	53,270	53,430
PPA [Member] | Derivative Liability (Current) [Member]
Derivative Fair Values [Abstract]
Liability Derivatives, fair value, gross basis	8,340	6,870
PPA [Member] | Derivative Liability (Long-term) [Member]
Derivative Fair Values [Abstract]
Liability Derivatives, fair value, gross basis	$ 44,930	$ 46,560