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Fair Value Measurements - Debt Securities, Trading, and Equity Securities, FV-NI (Details) - USD ($)
$ in Millions
12 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Deferred compensation plan assets    
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]    
Equity securities, FV-NI, unrealized gain (loss) $ 5 $ 10
Investments in exchange traded funds    
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]    
Equity securities, FV-NI, unrealized gain (loss) $ 29 $ 58