Derivative and Hedging Instruments (Details-1) $ in Thousands		9 Months Ended		12 Months Ended
		Sep. 30, 2017 USD ($) Dth MWh MW		Dec. 31, 2016 USD ($) Dth MWh MW	Jun. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)
Derivative fair values [Abstract]
Receivable, margin account balance, net of collateral posted against derivative positions		$ 1,500		$ 1,300
Other Current Assets [Member]
Derivative fair values [Abstract]
Derivative fair value, net		874		230	$ 618	$ 688	$ 659	$ 1,208
Commodity Contracts And Financial Transimission Rights [Member]
Derivative fair values [Abstract]
Derivative fair value, net		$ 900		$ 1,300
Commodity Derivative Contracts [Member]
Gross Notional Volume of Open Derivatives
Notional amount, energy measure (in MWh) | MWh		634,605		393,395
Notional amount, decatherm measure (in Dth) | Dth		7,924,500		4,195,000
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis		$ 969		$ 1,384
Liability Derivatives, fair value, gross basis		682		218
Commodity Derivative Contracts [Member] | Other Current Assets [Member]
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	[1]	857		1,227
Liability Derivatives, fair value, gross basis	[1]	560	[2]	164
Derivative, right to reclaim collateral (receivable)		0		0
Commodity Derivative Contracts [Member] | Other Deferred Charges [Member]
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	[1]	112		157
Liability Derivatives, fair value, gross basis	[1]	122		54
Derivative, right to reclaim collateral (receivable)		0		$ 0
Commodity Derivative Contracts [Member] | Derivative Liability (Current) [Member]
Derivative fair values [Abstract]
Liability Derivatives, fair value, gross basis		$ 100
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		3,552		2,251
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis		$ 567		$ 143
Financial Transmission Rights [Member] | Other Current Assets [Member]
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis		567		143
Liability Derivatives, fair value, gross basis		$ 0		$ 0
PPA [Member]
Gross Notional Volume of Open Derivatives
Notional amount, power measure (in MW) | MW		2,800		3,250
Derivative fair values [Abstract]
Derivative fair value, net		$ (43,100)		$ (50,600)
Liability Derivatives, fair value, gross basis		43,130		50,590
PPA [Member] | Derivative Liability (Current) [Member]
Derivative fair values [Abstract]
Liability Derivatives, fair value, gross basis		8,240		7,620
PPA [Member] | Derivative Liability (Long-term) [Member]
Derivative fair values [Abstract]
Liability Derivatives, fair value, gross basis		$ 34,890		$ 42,970

[1] As of September 30, 2017 , and December 31, 2016 , no collateral was posted against and netted with derivative liability positions on the consolidated balance sheets.

[2] As of September 30, 2017 , $0.1 million was presented as current derivative liabilities on the consolidated balance sheets.