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REGULATORY MATTERS: Oregon Base Rate Changes (Details) - USD ($)
$ in Thousands
12 Months Ended
Oct. 15, 2024
Dec. 31, 2026
Oct. 15, 2025
Dec. 31, 2024
Dec. 31, 2025
Jun. 01, 2025
Jun. 01, 2024
Jun. 01, 2023
Oregon Base Rate Changes [Line Items]                
Regulatory Assets       $ 1,507,372 $ 1,564,458      
Power supply costs                
Oregon Base Rate Changes [Line Items]                
Regulatory Assets [1]       $ 18,507 0      
Oregon jurisdiction                
Oregon Base Rate Changes [Line Items]                
Annual Power Cost Update           $ (1,800) $ (6,900) $ 7,700
OREGON | Wildfire Mitigation Plan Deferral                
Oregon Base Rate Changes [Line Items]                
Regulatory Assets         3,500      
OREGON | Subsequent Event                
Oregon Base Rate Changes [Line Items]                
Approved Rate Increase (Decrease), Amount   $ (600)            
Approved Rate Increase (Decrease), Percentage   0.90%            
OREGON | Subsequent Event | Valmy Plant                
Oregon Base Rate Changes [Line Items]                
Approved Rate Increase (Decrease), Amount   $ (1,200)            
OREGON | Subsequent Event | Boardman Plant                
Oregon Base Rate Changes [Line Items]                
Approved Rate Increase (Decrease), Amount   (100)            
OREGON | Subsequent Event | Wildfire mitigation plan costs                
Oregon Base Rate Changes [Line Items]                
Approved Rate Increase (Decrease), Amount   700            
OREGON | 2023 Rate Case                
Oregon Base Rate Changes [Line Items]                
Approved Rate Increase (Decrease), Amount     $ 6,700          
Approved Return on Equity, Percentage 9.50%              
Approved Rate Increase (Decrease), Percentage     12.14%          
OREGON | 2023 Rate Case | Cost of Capital and Capital Structure                
Oregon Base Rate Changes [Line Items]                
Authorized Rate of Return in Rate Case 7.302%              
Total Retail Rate Base $ 188,900              
OREGON | 2023 Rate Case | Cost of Debt Capital Structure [Domain]                
Oregon Base Rate Changes [Line Items]                
Approved Rate of Return 5.104%              
IDAHO | Wildfire Mitigation Plan Deferral                
Oregon Base Rate Changes [Line Items]                
Regulatory Assets         88,200      
IDAHO | 2023 Rate Case                
Oregon Base Rate Changes [Line Items]                
Approved Return on Equity, Percentage       9.60%        
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount       $ (54,700)        
Public Utilities, Requested Rate Increase (Decrease), Amended, Percentage       4.25%        
IDAHO | 2025 Rate Case | Wildfire Mitigation Plan Deferral                
Oregon Base Rate Changes [Line Items]                
Regulatory Assets         73,900      
IDAHO | 2025 Rate Case | Subsequent Event                
Oregon Base Rate Changes [Line Items]                
Approved Rate Increase (Decrease), Amount   $ 110,000            
Approved Return on Equity, Percentage   9.60%            
Approved Rate Increase (Decrease), Percentage   7.48%            
IDAHO | Wildfire mitigation plan costs | Wildfire Mitigation Plan Deferral                
Oregon Base Rate Changes [Line Items]                
Regulatory Assets         $ 700      
[1] This item is discussed in more detail in this Note 3 - "Regulatory Matters."