ORGANIZATION (Details) $ in Thousands, MWh in Millions	12 Months Ended
	Dec. 31, 2023 USD ($) employee MWh member_system non-utility_member class_of_membership	Dec. 31, 2022 USD ($) MWh	Dec. 31, 2021 USD ($) MWh
Concentration Risk [Line Items]
Number of classes of members | class_of_membership	3
Number of electric distribution member systems | member_system	42
Number of non-utility members | non-utility_member	3
Total megawatt-hours sold | MWh	18.2	18.6	17.6
Energy resources, proportion supplied by generation (as a percent)	48.20%	54.30%	52.10%
Number of employees | employee	1,120
Workforce Subject to Collective Bargaining Arrangements | Unionized Employees Concentration Risk
Concentration Risk [Line Items]
Number of employees | employee	215
Member electric sales
Concentration Risk [Line Items]
Revenue from contract with customer | $	$ 1,208,352	$ 1,213,234	$ 1,161,291
Electric sales
Concentration Risk [Line Items]
Revenue from contract with customer | $	$ 1,400,000	$ 1,400,000	$ 1,300,000
Utility Member | Member electric sales | Total Megawatt-Hours | Customer Concentration
Concentration Risk [Line Items]
Concentration risk (as a percent)	90.70%	88.80%	89.10%
Utility Member | Member electric sales | Electric Sales Provided by Generation | Customer Concentration
Concentration Risk [Line Items]
Concentration risk (as a percent)	86.20%	82.40%	86.40%