REVENUE - Summary (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Disaggregation of revenue
Other	$ 66,615	$ 61,420	$ 56,341
Operating revenues	1,467,416	1,533,571	1,400,801
Non-member electric sales and other operating revenue
Disaggregation of revenue
Operating revenues	259,064	320,337	239,510
Non-member electric sales
Disaggregation of revenue
Revenue from contract with customer	145,228	163,355	114,908
Non-member electric sales | Long-term contracts
Disaggregation of revenue
Revenue from contract with customer	43,087	56,570	44,383
Non-member electric sales | Short-term contracts
Disaggregation of revenue
Revenue from contract with customer	102,141	106,785	70,525
Rate stabilization
Disaggregation of revenue
Revenue from contract with customer	47,127	95,613	78,457
Provision for rate refunds
Disaggregation of revenue
Revenue from contract with customer	94	(51)	(10,196)
Coal sales
Disaggregation of revenue
Revenue from contract with customer	13,257	7,021	4,951
Other
Disaggregation of revenue
Other	$ 53,358	$ 54,399	$ 51,390