v3.26.1
REVENUE - Summary (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2026
Mar. 31, 2025
Disaggregation of Revenue [Line Items]    
Other $ 32,448 $ 28,405
Operating revenues 375,000 375,200
Total non-member electric sales and other operating revenue    
Disaggregation of Revenue [Line Items]    
Operating revenues 114,078 119,200
Non-member electric sales    
Disaggregation of Revenue [Line Items]    
Revenue from (refunds to) contract with customer 56,790 48,406
Rate stabilization    
Disaggregation of Revenue [Line Items]    
Revenue from (refunds to) contract with customer 24,840 44,943
Provision for rate refunds    
Disaggregation of Revenue [Line Items]    
Revenue from (refunds to) contract with customer 0 (2,554)
Lease revenue    
Disaggregation of Revenue [Line Items]    
Revenue from (refunds to) contract with customer 11,041 7,320
Transmission revenue    
Disaggregation of Revenue [Line Items]    
Revenue from (refunds to) contract with customer 19,159 17,610
Other    
Disaggregation of Revenue [Line Items]    
Other 2,248 3,475
Long-term contracts | Non-member electric sales    
Disaggregation of Revenue [Line Items]    
Revenue from (refunds to) contract with customer 46,822 38,794
Short-term contracts | Non-member electric sales    
Disaggregation of Revenue [Line Items]    
Revenue from (refunds to) contract with customer $ 9,968 $ 9,612