Derivative and Hedging Instruments (Details-1) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019 USD ($) Dth MWh MW	Dec. 31, 2018 USD ($) Dth MWh MW	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Derivative fair values [Abstract]
Receivable, margin account balance, net of collateral posted against derivative positions	$ 1,570	$ 1,193
Other Current Assets [Member]
Derivative fair values [Abstract]
Derivative fair value, net	259	377	$ 319	$ 806
Commodity Contracts And Financial Transimission Rights [Member]
Derivative fair values [Abstract]
Derivative fair value, net	$ 600	$ 700
Commodity Derivative Contracts [Member]
Gross Notional Volume of Open Derivatives
Notional amount, energy measure (in MWh) | MWh	440,440	386,440
Notional amount, decatherm measure (in Dth) | Dth	2,871,760	5,260,000
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	$ 675	$ 801
Liability Derivatives, fair value, gross basis	212	342
Commodity Derivative Contracts [Member] | Other Current Assets [Member]
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	546	727
Liability Derivatives, fair value, gross basis	114	270
Commodity Derivative Contracts [Member] | Other Deferred Charges [Member]
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	129	74
Liability Derivatives, fair value, gross basis	$ 98	$ 72
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	844	2,252
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	$ 124	$ 241
Financial Transmission Rights [Member] | Other Current Assets [Member]
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	124	241
Liability Derivatives, fair value, gross basis	$ 0	$ 0
PPA [Member]
Gross Notional Volume of Open Derivatives
Notional amount, power measure (in MW) | MW	1,900	2,050
Derivative fair values [Abstract]
Derivative fair value, net	$ (31,600)	$ (32,500)
Liability Derivatives, fair value, gross basis	31,610	32,530
PPA [Member] | Derivative Liability (Current) [Member]
Derivative fair values [Abstract]
Liability Derivatives, fair value, gross basis	8,730	8,550
PPA [Member] | Derivative Liability (Long-term) [Member]
Derivative fair values [Abstract]
Liability Derivatives, fair value, gross basis	$ 22,880	$ 23,980