Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Disaggregation of Revenue [Line Items]
Contract with Customer, Asset, before Allowance for Credit Loss	$ 166,400		$ 166,400		$ 163,800
Revenue from Contract with Customer, Excluding Assessed Tax	371,600	$ 350,500	1,043,800	$ 1,004,800
REVENUES	380,837	356,088	1,073,625	1,024,973
Contract with Customer, Liability	0		0		$ 500
Contract with Customer, Liability, Revenue Recognized	0	300	500	1,000
Retail Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	365,373	329,457	1,011,126	962,919
Other non-606 revenue	8,806	5,225	28,111	18,910
Wholesale Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	2,733	17,667	23,760	33,476
Miscellaneous revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	3,494	3,331	8,867	8,392
Other non-606 revenue	431	408	1,761	1,276
Electricity [Member]
Disaggregation of Revenue [Line Items]
REVENUES	380,837	356,088	1,073,625	1,024,973
Utility | Retail Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	4,206	3,875	12,572	10,568
Utility | Retail Revenue [Member] | Residential Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	158,806	145,407	448,563	428,568
Utility | Retail Revenue [Member] | Small Commercial and Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	56,220	50,023	158,945	147,678
Utility | Retail Revenue [Member] | Large Commercial and Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	143,871	127,884	384,320	369,224
Utility | Retail Revenue [Member] | Public Lighting
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 2,270	$ 2,268	$ 6,726	$ 6,881