REVENUE - Summary (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Disaggregation of Revenue [Line Items]
Other		$ 37,119	$ 15,618	$ 80,767	$ 48,162
Operating revenues		528,883	438,319	1,217,931	1,120,824
Total non-member electric sales and other operating revenue
Disaggregation of Revenue [Line Items]
Operating revenues		219,284	80,743	354,220	196,847
Non-member electric sales
Disaggregation of Revenue [Line Items]
Revenue from contract with customer		73,943	51,294	148,330	111,729
Rate stabilization
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	$ 76,400	108,222	14,041	125,123	36,862
Provision for rate refunds
Disaggregation of Revenue [Line Items]
Revenue from contract with customer		0	(210)	0	94
Coal sales
Disaggregation of Revenue [Line Items]
Revenue from contract with customer		7,856	3,675	11,314	8,069
Other
Disaggregation of Revenue [Line Items]
Other		29,263	11,943	69,453	40,093
Long-term contracts | Non-member electric sales
Disaggregation of Revenue [Line Items]
Revenue from contract with customer		35,481	9,731	86,669	31,527
Short-term contracts | Non-member electric sales
Disaggregation of Revenue [Line Items]
Revenue from contract with customer		$ 38,462	$ 41,563	$ 61,661	$ 80,202