Derivative and Hedging Instruments (Details-1) $ in Thousands		6 Months Ended		12 Months Ended
		Jun. 30, 2017 USD ($) Dth MWh MW		Dec. 31, 2016 USD ($) Dth MWh MW	Mar. 31, 2017 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Derivative fair values [Abstract]
Receivable, margin account balance, net of collateral posted against derivative positions		$ 1,600		$ 1,300
Other Current Assets [Member]
Derivative fair values [Abstract]
Derivative fair value, net		618		230	$ 142	$ 659	$ 655	$ 1,208
Commodity Contracts And Financial Transimission Rights [Member]
Derivative fair values [Abstract]
Derivative fair value, net		$ 1,300		$ 1,300
Commodity Derivative Contracts [Member]
Gross Notional Volume of Open Derivatives
Notional amount, energy measure (in MWh) | MWh		461,165		393,395
Notional amount, decatherm measure (in Dth) | Dth		4,781,500		4,195,000
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis		$ 817		$ 1,384
Liability Derivatives, fair value, gross basis		564		218
Commodity Derivative Contracts [Member] | Other Current Assets [Member]
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	[1]	807		1,227
Liability Derivatives, fair value, gross basis	[1]	459	[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]	10		157
Liability Derivatives, fair value, gross basis	[1]	105		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		5,091		2,251
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis		$ 1,001		$ 143
Financial Transmission Rights [Member] | Other Current Assets [Member]
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis		1,001		143
Liability Derivatives, fair value, gross basis		$ 0		$ 0
PPA [Member]
Gross Notional Volume of Open Derivatives
Notional amount, power measure (in MW) | MW		2,950		3,250
Derivative fair values [Abstract]
Derivative fair value, net		$ (46,600)		$ (50,600)
Liability Derivatives, fair value, gross basis		46,570		50,590
PPA [Member] | Derivative Liability (Current) [Member]
Derivative fair values [Abstract]
Liability Derivatives, fair value, gross basis		8,950		7,620
PPA [Member] | Derivative Liability (Long-term) [Member]
Derivative fair values [Abstract]
Liability Derivatives, fair value, gross basis		$ 37,620		$ 42,970

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