Derivative and Hedging Instruments (Details-1) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2016 USD ($) Dth MWh MW	Dec. 31, 2015 USD ($) Dth MWh MW	Jun. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)
Gross Notional Volume of Open Derivatives
Derivative, right to reclaim collateral (receivable)	$ 0
Receivable, margin account balance, net of collateral posted against derivative positions	1,600	$ 2,300
Other Current Assets [Member]
Derivative fair values [Abstract]
Derivative fair value, net	688	1,208	$ 659	$ 772	$ 455	$ 1,001
Commodity Contracts And Financial Transimission Rights [Member]
Derivative fair values [Abstract]
Derivative fair value, net	$ 900	$ (800)
Commodity Derivative Contracts [Member]
Gross Notional Volume of Open Derivatives
Notional amount, energy measure (in MWh) | MWh	465,835	355,580
Notional amount, decatherm measure (in Dth) | Dth	6,157,500	5,037,500
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	$ 1,137	$ 290
Liability Derivatives, fair value, gross basis	448	1,336
Commodity Derivative Contracts [Member] | Other Current Assets [Member]
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	1,023	146
Commodity Derivative Contracts [Member] | Other Deferred Charges [Member]
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	114	144
Commodity Derivative Contracts [Member] | Derivative Liability (Current) [Member]
Derivative fair values [Abstract]
Liability Derivatives, fair value, gross basis	351	1,266
Commodity Derivative Contracts [Member] | Derivative Liability (Long-term) [Member]
Derivative fair values [Abstract]
Liability Derivatives, fair value, gross basis	$ 97	$ 70
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	3,567	2,000
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	$ 230	$ 234
Financial Transmission Rights [Member] | Other Current Assets [Member]
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	230	234
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	(50,500)	(53,300)
Liability Derivatives, fair value, gross basis	50,490	53,270
PPA [Member] | Derivative Liability (Current) [Member]
Derivative fair values [Abstract]
Liability Derivatives, fair value, gross basis	8,030	8,340
PPA [Member] | Derivative Liability (Long-term) [Member]
Derivative fair values [Abstract]
Liability Derivatives, fair value, gross basis	$ 42,460	$ 44,930