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Regulatory Assets and Liabilities (Details)
$ in Millions
Mar. 31, 2025
USD ($)
Sep. 30, 2024
USD ($)
Regulatory Assets and Liabilities    
Current regulatory assets $ 241 $ 191
Non-current regulatory assets 9,414 9,408
Environmental Remediation Costs Recognized [Abstract] 276 215
Regulatory assets 9,655 9,599
Current regulatory liabilities 287 174
Regulatory Liability, Noncurrent 81 83
Regulatory Liabilities 368 257
Fair value $ 13 $ 64
Commodity Contract Derivatives    
Regulatory Assets and Liabilities    
Number of contracts 39 45
Derivative, Nonmonetary Notional Amount 468,000,000 321,000,000
Fair value $ 5 $ 2
Fuel cost adjustment tax equivalents    
Regulatory Assets and Liabilities    
Current regulatory liabilities 179 169
Unrealized gains/losses on commodity derivatives    
Regulatory Assets and Liabilities    
Current regulatory liabilities 108 5
Regulatory Liability, Noncurrent 8 2
Postretirement Benefit Costs    
Regulatory Assets and Liabilities    
Regulatory Liability, Noncurrent 73 81
Unrealized losses on interest rate derivatives [Member]    
Regulatory Assets and Liabilities    
Current regulatory assets 52 54
Non-current regulatory assets 323 447
Unrealized gains/losses on commodity derivatives    
Regulatory Assets and Liabilities    
Current regulatory assets 3 102
Non-current regulatory assets 13 64
Fuel cost adjustment receivable/liability    
Regulatory Assets and Liabilities    
Current regulatory assets 186 35
Pension Costs [Member]    
Regulatory Assets and Liabilities    
Non-current regulatory assets 1,937 1,979
Nuclear desommissioning costs [Member]    
Regulatory Assets and Liabilities    
Non-current regulatory assets 477 362
Non-nuclear decommissioning [Member]    
Regulatory Assets and Liabilities    
Non-current regulatory assets 6,224 6,187
Other non-current regulatory assets [Member]    
Regulatory Assets and Liabilities    
Non-current regulatory assets $ 164 $ 154