PRESENTATION OF FINANCIAL INFORMATION - Narrative (Details) $ in Thousands		1 Months Ended	3 Months Ended		9 Months Ended
	May 01, 2024 USD ($)	Sep. 30, 2024 USD ($)	Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)	Sep. 30, 2024 USD ($) non-utility_member member_system class_of_membership	Sep. 30, 2023 USD ($)	Dec. 31, 2023 USD ($)
Disaggregation of Revenue [Line Items]
Number of classes of members | class_of_membership					3
Regulatory assets		$ 826,476	$ 826,476		$ 826,476		$ 919,483
Goodwill impairment			68,223	$ 0	68,223	$ 0
Amount received for contract termination payment	$ 709,400
Exit fee related to contract termination payment	627,200
Patronage capital credit related to contract termination payment	82,200
Regulatory liability		584,375	584,375		584,375		2,317
Transmission credit portion of contract termination payment	179,300
Deferred membership withdrawal income
Disaggregation of Revenue [Line Items]
Regulatory liability	$ 530,100	405,478	$ 405,478		$ 405,478		$ 463
J.M. Shafer and Colowyo Coal | Acquisition costs
Disaggregation of Revenue [Line Items]
Goodwill impairment		$ 68,200
Class A Members
Disaggregation of Revenue [Line Items]
Number of members | member_system					41
Class B Members
Disaggregation of Revenue [Line Items]
Number of members | member_system					0
Non-Utility Members
Disaggregation of Revenue [Line Items]
Number of members | non-utility_member					3