REVENUE - Summary (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Disaggregation of Revenue [Line Items]
Other	$ 23,544	$ 14,686	$ 43,648	$ 32,544
Operating revenues	337,499	318,455	689,048	682,506
Total non-member electric sales and other operating revenue
Disaggregation of Revenue [Line Items]
Operating revenues	79,213	44,230	134,936	116,105
Non-member electric sales
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	38,768	18,486	74,387	60,436
Rate stabilization
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	16,901	11,064	16,901	22,821
Provision for rate refunds
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	(6)	0	304
Coal sales
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,814	3,315	3,458	4,394
Other
Disaggregation of Revenue [Line Items]
Other	21,730	11,371	40,190	28,150
Long-term contracts | Non-member electric sales
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	29,813	8,996	51,188	21,796
Short-term contracts | Non-member electric sales
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	$ 8,955	$ 9,490	$ 23,199	$ 38,640