Derivative and Hedging Instruments (Details-1) $ in Thousands		12 Months Ended
		Dec. 31, 2017 USD ($) Dth MWh MW	Dec. 31, 2016 USD ($) Dth MWh MW		Dec. 31, 2015 USD ($)
Derivative Fair Values [Abstract]
Derivative, right to reclaim collateral (receivable)		$ 100	$ 0
Other Current Assets [Member]
Derivative Fair Values [Abstract]
Derivative fair value, net		806	230		$ 1,208
Commodity Contracts and Financial Transimission Rights [Member]
Derivative Fair Values [Abstract]
Derivative fair value, net		$ 200	$ 1,300
Commodity Derivative Contracts [Member]
Gross Notional Volume of Open Derivatives
Notional amount, energy measure (in MWh) | MWh		552,310	393,395
Notional amount, decatherm measure (in Dth) | Dth		5,460,000	4,195,000
Derivative Fair Values [Abstract]
Asset Derivatives, fair value, gross basis		$ 676	$ 1,384
Liability Derivatives, fair value, gross basis		793	218
Commodity Derivative Contracts [Member] | Other Current Assets [Member]
Derivative Fair Values [Abstract]
Asset Derivatives, fair value, gross basis	[1]		1,227
Liability Derivatives, fair value, gross basis		100	164	[1]
Commodity Derivative Contracts [Member] | Other Deferred Charges [Member]
Derivative Fair Values [Abstract]
Asset Derivatives, fair value, gross basis	[1]		157
Liability Derivatives, fair value, gross basis	[1]		$ 54
Commodity Derivative Contracts [Member] | Derivative Liability (Current) [Member]
Derivative Fair Values [Abstract]
Asset Derivatives, fair value, gross basis	[1],[2]	566
Liability Derivatives, fair value, gross basis	[1],[2]	603
Commodity Derivative Contracts [Member] | Derivative Liability (Long-term) [Member]
Derivative Fair Values [Abstract]
Asset Derivatives, fair value, gross basis	[1]	110
Liability Derivatives, fair value, gross basis	[1]	$ 190
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		2,226	2,251
Derivative Fair Values [Abstract]
Asset Derivatives, fair value, gross basis		$ 329	$ 143
Financial Transmission Rights [Member] | Other Current Assets [Member]
Derivative Fair Values [Abstract]
Asset Derivatives, fair value, gross basis		329	143
Liability Derivatives, fair value, gross basis		$ 0	$ 0
PPA [Member]
Gross Notional Volume of Open Derivatives
Notional amount, power measure (in MW) | MW		2,650	3,250
Derivative Fair Values [Abstract]
Derivative fair value, net		$ (42,200)	$ (50,600)
Liability Derivatives, fair value, gross basis		42,170	50,590
PPA [Member] | Derivative Liability (Current) [Member]
Derivative Fair Values [Abstract]
Liability Derivatives, fair value, gross basis		8,180	7,620
PPA [Member] | Derivative Liability (Long-term) [Member]
Derivative Fair Values [Abstract]
Liability Derivatives, fair value, gross basis		$ 33,990	$ 42,970

[1] As of December 31, 2017 , collateral of $ 0.1 million was posted against and netted with derivative liability positions on the consolidated balance sheets. No collateral was posted against derivative positions as of December 31, 2016 .

[2] As of December 31, 2017 , $ 0.1 million was presented as other current assets on the consolidated balance sheets.