v3.26.1
ACCOUNTING FOR RATE REGULATION (Details) - USD ($)
$ in Thousands
 1 Months Ended 3 Months Ended 9 Months Ended
Oct. 31, 2017
Mar. 31, 2026
Mar. 31, 2025
Sep. 30, 2025
Dec. 31, 2025
Dec. 31, 2024
Regulatory Asset and Liability [Line Items]            
Regulatory assets   $ 851,785     $ 853,834  
Regulatory liabilities   331,339     358,446  
Net regulatory asset   520,446     495,388  
Rate stabilization            
Regulatory Asset and Liability [Line Items]            
Revenue from (refunds to) contract with customer   $ 24,840 $ 44,943      
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   $ 829     924  
Proceeds from settlement of interest rate swaps $ 4,600          
Deferred Membership Withdrawal Income            
Regulatory Asset and Liability [Line Items]            
Regulatory liabilities   5,396     30,236  
Transmission credit            
Regulatory Asset and Liability [Line Items]            
Regulatory liabilities   313,731     315,786  
Formula rate settlement            
Regulatory Asset and Liability [Line Items]            
Regulatory liabilities   11,129     11,500  
Revenue Subject to Refund | Rate stabilization            
Regulatory Asset and Liability [Line Items]            
Revenue from (refunds to) contract with customer   24,800        
Deferred income tax expense            
Regulatory Asset and Liability [Line Items]            
Regulatory liabilities   254     0  
Deferred income tax expense            
Regulatory Asset and Liability [Line Items]            
Regulatory assets   0     (254)  
Deferred prepaid lease expense | Springerville Unit 3 Lease            
Regulatory Asset and Liability [Line Items]            
Regulatory assets   69,397     69,970  
Annual amortization expense of regulatory asset   $ 2,300        
Amortization period   47 years        
Deferred debt prepayment transaction costs            
Regulatory Asset and Liability [Line Items]            
Regulatory assets   $ 87,003     89,160  
Annual amortization expense of regulatory asset   $ 8,600        
Amortization period   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   64,277     65,446  
Annual amortization expense of regulatory asset   $ 4,700        
Amortization period   20 years        
Deferred impairment loss | J.M. Shafer            
Regulatory Asset and Liability [Line Items]            
Amortization period   25 years        
Environmental obligation            
Regulatory Asset and Liability [Line Items]            
Regulatory assets   $ 41,878     42,326 $ 44,900
Annual amortization expense of regulatory asset   $ 1,800        
Amortization period   25 years        
Asset retirement obligation            
Regulatory Asset and Liability [Line Items]            
Regulatory assets   $ 85,100     79,860  
Annual amortization expense of regulatory asset   $ 4,500        
Amortization period   19 years        
Unrecovered plant | Unrecovered Plants            
Regulatory Asset and Liability [Line Items]            
Regulatory assets   $ 504,054     507,250  
Other            
Regulatory Asset and Liability [Line Items]            
Regulatory assets   $ 76     $ 76