Derivative and Hedging Instruments (Details-1) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2016 USD ($) Dth MWh MW	Dec. 31, 2015 USD ($) Dth MWh MW	Mar. 31, 2016 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Other Current Assets [Member]
Derivative fair values [Abstract]
Derivative fair value, net	$ 659	$ 1,208	$ 655	$ 455	$ 268	$ 1,001
Commodity Contracts And Financial Transimission Rights [Member]
Derivative fair values [Abstract]
Derivative fair value, net	$ 1,700	$ (800)
Commodity Derivative Contracts [Member]
Gross Notional Volume of Open Derivatives
Notional amount, energy measure (in MWh) | MWh	479,725	355,580
Notional amount, decatherm measure (in Dth) | Dth	1,887,500	5,037,500
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	$ 1,957	$ 290
Liability Derivatives, fair value, gross basis	272	1,336
Commodity Derivative Contracts [Member] | Other Current Assets [Member]
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	1,814	146
Commodity Derivative Contracts [Member] | Other Deferred Charges [Member]
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	143	144
Commodity Derivative Contracts [Member] | Derivative Liability (Current) [Member]
Derivative fair values [Abstract]
Liability Derivatives, fair value, gross basis	212	1,266
Commodity Derivative Contracts [Member] | Derivative Liability (Long-term) [Member]
Derivative fair values [Abstract]
Liability Derivatives, fair value, gross basis	$ 60	$ 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	5,653	2,000
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	$ 34	$ 234
Financial Transmission Rights [Member] | Other Current Assets [Member]
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	34	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	(52,200)	(53,300)
Liability Derivatives, fair value, gross basis	52,240	53,270
PPA [Member] | Derivative Liability (Current) [Member]
Derivative fair values [Abstract]
Liability Derivatives, fair value, gross basis	8,650	8,340
PPA [Member] | Derivative Liability (Long-term) [Member]
Derivative fair values [Abstract]
Liability Derivatives, fair value, gross basis	$ 43,590	$ 44,930