v3.26.1
NATURE OF OPERATIONS AND FINANCIAL STATEMENT PRESENTATION (Narrative) (Details)
customer in Thousands, $ in Millions
1 Months Ended 3 Months Ended
Apr. 09, 2026
USD ($)
Dec. 31, 2025
USD ($)
Jul. 31, 2025
USD ($)
Mar. 31, 2026
USD ($)
mi²
customer
operatingSegment
Mar. 31, 2025
USD ($)
Public Utilities, General Disclosures [Line Items]          
Number of retail customers | customer       458  
Area in which company generates transmits and distributes electricity to retail electric customers (sq miles) | mi²       1,155  
Number of operating segments | operatingSegment       1  
Operations and maintenance expense, excluding customer funded expenses       $ 92 $ 82
PTC benefits       2 $ 4
Roadrunner Reserve II          
Public Utilities, General Disclosures [Line Items]          
Accumulated deferred investment tax credit       10  
Valuation allowance       $ 1  
Roadrunner Reserve I          
Public Utilities, General Disclosures [Line Items]          
Investment tax credit     $ 98    
Sales of investment tax credits   $ 49      
Proceeds from sale of investment tax credit   $ 46      
Roadrunner Reserve I | Subsequent Event          
Public Utilities, General Disclosures [Line Items]          
Sales of investment tax credits $ 49        
Proceeds from sale of investment tax credit $ 46