Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Disaggregation of Revenue [Line Items]
Contract with Customer, Asset, before Allowance for Credit Loss	$ 149,800		$ 149,800		$ 163,800
Revenue from Contract with Customer, Excluding Assessed Tax	318,500	$ 302,400	672,200	$ 654,300
REVENUES	330,587	311,503	692,788	668,885
Contract with Customer, Liability	0		0		$ 500
Contract with Customer, Liability, Revenue Recognized	200	300	500	700
Retail Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	310,916	295,570	645,753	633,463
Other non-606 revenue	11,710	8,789	19,305	13,684
Wholesale Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	4,918	4,501	21,027	15,809
Miscellaneous revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	2,692	2,329	5,373	5,061
Other non-606 revenue	351	314	1,330	868
Electricity [Member]
Disaggregation of Revenue [Line Items]
REVENUES	330,587	311,503	692,788	668,885
Utility | Retail Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	4,175	2,805	8,366	6,693
Utility | Retail Revenue [Member] | Residential Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	129,254	127,752	289,757	283,161
Utility | Retail Revenue [Member] | Small Commercial and Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	48,208	43,402	102,725	97,655
Utility | Retail Revenue [Member] | Large Commercial and Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	127,009	119,302	240,449	241,340
Utility | Retail Revenue [Member] | Public Lighting
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 2,270	$ 2,309	$ 4,456	$ 4,614