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Accumulated Other Comprehensive Income (Loss) - Amounts Reclassified Out of AOCI (Detail) - USD ($) $ in Millions | 12 Months Ended
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Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2023
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] |  |  |
Total before tax | $ (1) | $ (5) | $ (6)
(Gains) Losses Reclassified to Interest Expense [Member] |  |  |
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] |  |  |
Total before tax | (5) | (5) | (6)
(Gains) Losses Excluded from Effectiveness Testing and Amortized to Interest Expense [Member] |  |  |
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] |  |  |
Total before tax | 2 | 2 | 2
Amortization of Actuarial Losses and Prior Service Cost (Credit) [Member] |  |  |
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] |  |  |
Total before tax | $ 2 | $ (2) | $ (2)

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