v3.26.3
ACCOUNTING FOR RATE REGULATION (Details) - USD ($)
$ in Thousands
 1 Months Ended 3 Months Ended 9 Months Ended
Oct. 31, 2017
Sep. 30, 2026
Mar. 31, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
Dec. 31, 2025
Dec. 31, 2024
Regulatory Asset and Liability [Line Items]                
Regulatory assets   $ 824,285     $ 824,285   $ 853,834  
Regulatory liabilities   510,741     510,741   358,446  
Net regulatory asset   313,544     313,544   495,388  
Rate stabilization                
Regulatory Asset and Liability [Line Items]                
Revenue from (refunds to) contract with customer   16,491   $ 57,343 $ 50,700 $ 145,469    
First Mortgage Obligations, Series 2017A , Tranche 1, 3.34%, due through 2029                
Regulatory Asset and Liability [Line Items]                
Term of issuance         12 years      
Craig Generating Station Unit 2 And 3                
Regulatory Asset and Liability [Line Items]                
Impairment losses of long-lived assets     $ 261,600          
Interest rate swaps, realized gain                
Regulatory Asset and Liability [Line Items]                
Regulatory liabilities   641     $ 641   924  
Proceeds from settlement of interest rate swaps $ 4,600              
Deferred Membership Withdrawal Income                
Regulatory Asset and Liability [Line Items]                
Regulatory liabilities   99,104     99,104   30,236  
Transmission credit                
Regulatory Asset and Liability [Line Items]                
Regulatory liabilities   397,378     397,378   315,786  
Formula rate settlement                
Regulatory Asset and Liability [Line Items]                
Regulatory liabilities   10,115     10,115   11,500  
Revenue Subject to Refund | Rate stabilization                
Regulatory Asset and Liability [Line Items]                
Revenue from (refunds to) contract with customer         50,700      
Deferred income tax expense                
Regulatory Asset and Liability [Line Items]                
Regulatory liabilities   3,503     3,503   0  
Deferred income tax expense                
Regulatory Asset and Liability [Line Items]                
Regulatory assets   0     0   (254)  
Deferred prepaid lease expense | Springerville Unit 3 Lease                
Regulatory Asset and Liability [Line Items]                
Regulatory assets   $ 68,252     68,252   69,970  
Annual amortization expense of regulatory asset         $ 2,300      
Amortization period   47 years     47 years      
Deferred debt prepayment transaction costs                
Regulatory Asset and Liability [Line Items]                
Regulatory assets   $ 82,689     $ 82,689   89,160  
Annual amortization expense of regulatory asset         $ 8,600      
Amortization period   21 years 4 months 24 days     21 years 4 months 24 days      
Deferred impairment loss | Escalante Generating Station                
Regulatory Asset and Liability [Line Items]                
Annual amortization expense of regulatory asset         $ 12,200      
Deferred impairment loss | Rifle Generating Station                
Regulatory Asset and Liability [Line Items]                
Annual amortization expense of regulatory asset         600      
Deferred impairment loss | Holcomb Expansion                
Regulatory Asset and Liability [Line Items]                
Regulatory assets   $ 61,940     61,940   65,446  
Annual amortization expense of regulatory asset         $ 4,700      
Amortization period   20 years     20 years      
Deferred impairment loss | J.M. Shafer                
Regulatory Asset and Liability [Line Items]                
Amortization period   25 years     25 years      
Environmental obligation                
Regulatory Asset and Liability [Line Items]                
Regulatory assets   $ 40,980     $ 40,980   42,326 $ 44,900
Annual amortization expense of regulatory asset         $ 1,800      
Amortization period   25 years     25 years      
Asset retirement and environmental remediation obligations costs                
Regulatory Asset and Liability [Line Items]                
Regulatory assets   $ 72,427     $ 72,427   79,860  
Annual amortization expense of regulatory asset         $ 4,000      
Amortization period   19 years     19 years      
Unrecovered plant | Unrecovered Plants                
Regulatory Asset and Liability [Line Items]                
Regulatory assets   $ 497,662     $ 497,662   507,250  
Other                
Regulatory Asset and Liability [Line Items]                
Regulatory assets   $ 335     $ 335   $ 76