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Fair Value Measurements - Debt Securities, Trading, and Equity Securities, FV-NI (Details) - USD ($) $ in Millions | 12 Months Ended
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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

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