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Risk Management Activities and Derivative Transactions Risk Management Activities and Derivative Transactions - Offsetting of Derivative Liabilities (Details) - USD ($)
$ in Millions
Sep. 30, 2025
Sep. 30, 2024
Offsetting Liabilities [Line Items]    
Gross Amounts Offset in the Balance Sheet $ 20 $ 4
Amount of letters of credit outstanding 6  
Derivative Liability, Subject to Master Netting Arrangement, after Offset 884 1,122
Derivative Asset, Subject to Master Netting Arrangement, after Offset 24 7
Commodity contract derivatives 16 7
Currency Swap    
Offsetting Liabilities [Line Items]    
Derivative Liability, Subject to Master Netting Arrangement, after Offset 115 116
Commodity Contract under FHP    
Offsetting Liabilities [Line Items]    
Gross Amounts of Recognized Liabilities, subject to master netting or similar arrangements 85 165
Derivative Liability, Subject to Master Netting Arrangement, after Offset 65 161
Derivative Asset, Subject to Master Netting Arrangement, after Offset 8 0
Interest Rate Swap    
Offsetting Liabilities [Line Items]    
Derivative Liability, Subject to Master Netting Arrangement, after Offset 698 840
Commodity Contract Derivatives    
Offsetting Liabilities [Line Items]    
Derivative Liability, Subject to Master Netting Arrangement, after Offset 6 5
Derivative Asset, Subject to Master Netting Arrangement, after Offset 16 7
Letter of Credit    
Offsetting Liabilities [Line Items]    
Amount of letters of credit outstanding $ 498 566
Interest rate swap collateral [Member]    
Offsetting Liabilities [Line Items]    
Amount of letters of credit outstanding   $ 535