Derivative and Hedging Instruments (Details-1) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2015 USD ($) Dth MWh MW	Dec. 31, 2014 USD ($) Dth MWh MW	Jun. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Dec. 31, 2013 USD ($)
Gross Notional Volume of Open Derivatives
Derivative, right to reclaim collateral (receivable)	$ 800
Other Current Assets [Member]
Derivative fair values [Abstract]
Derivative fair value, net	772	$ 1,001	$ 455	$ 741	$ 676	$ 411
Commodity Contracts And Financial Transimission Rights [Member]
Derivative fair values [Abstract]
Derivative fair value, net	$ (400)	$ (1,600)
Commodity Derivative Contracts [Member]
Gross Notional Volume of Open Derivatives
Notional amount, energy measure (in MWh) | MWh	343,560	448,000
Notional amount, decatherm measure (in Dth) | Dth	6,547,500	4,405,000
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	$ 440	$ 223
Liability Derivatives, fair value, gross basis	1,258	2,433
Commodity Derivative Contracts [Member] | Other Current Assets [Member]
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	375	130
Commodity Derivative Contracts [Member] | Other Deferred Charges [Member]
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	65	93
Commodity Derivative Contracts [Member] | Derivative Liability (Current) [Member]
Derivative fair values [Abstract]
Liability Derivatives, fair value, gross basis	1,196	2,262
Commodity Derivative Contracts [Member] | Derivative Liability (Long-term) [Member]
Derivative fair values [Abstract]
Liability Derivatives, fair value, gross basis	$ 62	$ 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	3,286	1,854
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	$ 370	$ 642
Financial Transmission Rights [Member] | Other Current Assets [Member]
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	370	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	(57,000)	(53,400)
Liability Derivatives, fair value, gross basis	56,950	53,430
PPA [Member] | Derivative Liability (Current) [Member]
Derivative fair values [Abstract]
Liability Derivatives, fair value, gross basis	8,880	6,870
PPA [Member] | Derivative Liability (Long-term) [Member]
Derivative fair values [Abstract]
Liability Derivatives, fair value, gross basis	$ 48,070	$ 46,560