Risk Management Activities and Derivative Transactions Risk Management Activities and Derivative Transactions - Offsetting of Derivative Assets (Details) - USD ($) $ in Millions	Sep. 30, 2025	Sep. 30, 2024
Offsetting Assets [Line Items]
Amount of letters of credit outstanding	$ 6
Letter of Credit
Offsetting Assets [Line Items]
Amount of letters of credit outstanding	498	$ 566
Commodity Contract under FHP
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets, subject to master netting or similar arrangements	$ 28	$ 4