v3.26.3
REVENUE - Summary (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
Disaggregation of Revenue [Line Items]        
Other $ 65,263 $ 39,906 $ 170,528 $ 107,836
Operating revenues 476,197 458,306 1,247,650 1,223,633
Total non-member electric sales and other operating revenue        
Disaggregation of Revenue [Line Items]        
Operating revenues 145,987 151,738 386,055 403,416
Non-member electric sales        
Disaggregation of Revenue [Line Items]        
Revenue from (refunds to) contract with customer 92,553 54,489 204,720 147,392
Rate stabilization        
Disaggregation of Revenue [Line Items]        
Revenue from (refunds to) contract with customer 16,491 57,343 50,700 145,469
Provision for rate refunds        
Disaggregation of Revenue [Line Items]        
Revenue from (refunds to) contract with customer (28,320) 0 (39,893) 2,719
Transmission revenue        
Disaggregation of Revenue [Line Items]        
Revenue from (refunds to) contract with customer 51,633 25,771 128,746 60,769
Lease revenue        
Disaggregation of Revenue [Line Items]        
Revenue from (refunds to) contract with customer 12,602 7,771 36,529 22,329
Coal sales        
Disaggregation of Revenue [Line Items]        
Revenue from (refunds to) contract with customer 0 1,249 0 13,714
Other        
Disaggregation of Revenue [Line Items]        
Other 1,028 5,115 5,253 11,024
Long-term contracts | Non-member electric sales        
Disaggregation of Revenue [Line Items]        
Revenue from (refunds to) contract with customer 44,766 39,011 136,900 114,284
Short-term contracts | Non-member electric sales        
Disaggregation of Revenue [Line Items]        
Revenue from (refunds to) contract with customer $ 47,787 $ 15,478 $ 67,820 $ 33,108