Derivative and Hedging Instruments (Details-1) $ in Thousands		3 Months Ended	12 Months Ended
		Mar. 31, 2018 USD ($) Dth MWh MW	Dec. 31, 2017 USD ($) Dth MWh MW	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Derivative fair values [Abstract]
Receivable, margin account balance, net of collateral posted against derivative positions		$ 2,325	$ 2,507
Other Current Assets [Member]
Derivative fair values [Abstract]
Derivative fair value, net		319	806	$ 142	$ 230
Commodity Contracts And Financial Transimission Rights [Member]
Derivative fair values [Abstract]
Derivative fair value, net		$ 100	$ 200
Commodity Derivative Contracts [Member]
Gross Notional Volume of Open Derivatives
Notional amount, energy measure (in MWh) | MWh		538,050	552,310
Notional amount, decatherm measure (in Dth) | Dth		3,270,000	5,460,000
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis		$ 412	$ 676
Liability Derivatives, fair value, gross basis		483	793
Commodity Derivative Contracts [Member] | Other Current Assets [Member]
Derivative fair values [Abstract]
Liability Derivatives, fair value, gross basis		100	100
Derivative, right to reclaim collateral (receivable)		0	0
Commodity Derivative Contracts [Member] | Derivative Liability (Current) [Member]
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	[1],[2]	403	566
Liability Derivatives, fair value, gross basis	[1],[2]	294	603
Commodity Derivative Contracts [Member] | Derivative Liability (Long-term) [Member]
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	[1]	9	110
Liability Derivatives, fair value, gross basis	[1]	$ 189	$ 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		886	2,226
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis		$ 120	$ 329
Financial Transmission Rights [Member] | Other Current Assets [Member]
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis		120	329
Liability Derivatives, fair value, gross basis		$ 0	$ 0
PPA [Member]
Gross Notional Volume of Open Derivatives
Notional amount, power measure (in MW) | MW		2,500	2,650
Derivative fair values [Abstract]
Derivative fair value, net		$ (41,300)	$ (42,200)
Liability Derivatives, fair value, gross basis		41,290	42,170
PPA [Member] | Derivative Liability (Current) [Member]
Derivative fair values [Abstract]
Liability Derivatives, fair value, gross basis		8,950	8,180
PPA [Member] | Derivative Liability (Long-term) [Member]
Derivative fair values [Abstract]
Liability Derivatives, fair value, gross basis		$ 32,340	$ 33,990

[1] As of March 31, 2018 , and December 31, 2017 , collateral of $0.1 million was posted against and netted with derivative liability positions on the consolidated balance sheets.

[2] As of March 31, 2018 , and December 31, 2017 , $0.1 million was presented as other current assets on the consolidated balance sheets.