Derivative and Hedging Instruments (Details-1) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2016 USD ($) Dth MWh MW	Dec. 31, 2015 USD ($) Dth MWh MW	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Gross Notional Volume of Open Derivatives
Derivative, right to reclaim collateral (receivable)	$ 1,000
Other Current Assets [Member]
Derivative fair values [Abstract]
Derivative fair value, net	655	$ 1,208	$ 268	$ 1,001
Commodity Contracts And Financial Transimission Rights [Member]
Derivative fair values [Abstract]
Derivative fair value, net	$ (1,000)	$ (800)
Commodity Derivative Contracts [Member]
Gross Notional Volume of Open Derivatives
Notional amount, energy measure (in MWh) | MWh	468,590	355,580
Notional amount, decatherm measure (in Dth) | Dth	2,317,500	5,037,500
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	$ 494	$ 290
Liability Derivatives, fair value, gross basis	1,487	1,336
Commodity Derivative Contracts [Member] | Other Current Assets [Member]
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	441	146
Commodity Derivative Contracts [Member] | Other Deferred Charges [Member]
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	53	144
Commodity Derivative Contracts [Member] | Derivative Liability (Current) [Member]
Derivative fair values [Abstract]
Liability Derivatives, fair value, gross basis	1,303	1,266
Commodity Derivative Contracts [Member] | Derivative Liability (Long-term) [Member]
Derivative fair values [Abstract]
Liability Derivatives, fair value, gross basis	$ 184	$ 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	770	2,000
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	$ 19	$ 234
Financial Transmission Rights [Member] | Other Current Assets [Member]
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	19	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	(55,900)	(53,300)
Liability Derivatives, fair value, gross basis	55,910	53,270
PPA [Member] | Derivative Liability (Current) [Member]
Derivative fair values [Abstract]
Liability Derivatives, fair value, gross basis	9,370	8,340
PPA [Member] | Derivative Liability (Long-term) [Member]
Derivative fair values [Abstract]
Liability Derivatives, fair value, gross basis	$ 46,540	$ 44,930