Derivative and Hedging Instruments (Details-1) $ in Thousands	6 Months Ended		12 Months Ended
	Jun. 30, 2019 USD ($) Dth MWh MW		Dec. 31, 2018 USD ($) Dth MWh MW	Mar. 31, 2019 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Derivative fair values [Abstract]
Receivable, margin account balance, net of collateral posted against derivative positions	$ 1,304		$ 1,193
Other Current Assets [Member]
Derivative fair values [Abstract]
Derivative fair value, net	878		377	$ 259	$ 640	$ 319	$ 806
Commodity Contracts And Financial Transimission Rights [Member]
Derivative fair values [Abstract]
Derivative fair value, net	$ (900)		$ 700
Commodity Derivative Contracts [Member]
Gross Notional Volume of Open Derivatives
Notional amount, energy measure (in MWh) | MWh	507,720		386,440
Notional amount, decatherm measure (in Dth) | Dth	7,140,000		5,260,000
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	$ 659		$ 801
Liability Derivatives, fair value, gross basis	2,035		342
Collateral posted against derivative positions	0		0
Commodity Derivative Contracts [Member] | Other Current Assets [Member]
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	578		727
Liability Derivatives, fair value, gross basis	1,322	[1]	270
Commodity Derivative Contracts [Member] | Other Deferred Charges [Member]
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	81		74
Liability Derivatives, fair value, gross basis	$ 713	[1]	$ 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	5,853		2,252
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	$ 430		$ 241
Collateral posted against derivative positions	0		0
Financial Transmission Rights [Member] | Other Current Assets [Member]
Derivative fair values [Abstract]
Asset Derivatives, fair value, gross basis	430		241
Liability Derivatives, fair value, gross basis	$ 0		$ 0
PPA [Member]
Gross Notional Volume of Open Derivatives
Notional amount, power measure (in MW) | MW	1,750		2,050
Derivative fair values [Abstract]
Derivative fair value, net	$ (32,600)		$ (32,500)
Liability Derivatives, fair value, gross basis	32,570		32,530
PPA [Member] | Derivative Liability (Current) [Member]
Derivative fair values [Abstract]
Liability Derivatives, fair value, gross basis	10,670		8,550
PPA [Member] | Derivative Liability (Long-term) [Member]
Derivative fair values [Abstract]
Liability Derivatives, fair value, gross basis	$ 21,900		$ 23,980

[1] As of June 30, 2019, collateral of $0 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, 2018.