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Accumulated Other Comprehensive Income (Loss) ("AOCI") - Amounts Reclassified Out of AOCI (Detail) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Jun. 30, 2025 | Jun. 30, 2024 | Jun. 30, 2025 | Jun. 30, 2024
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] |  |  |  |
Total before tax |  |  | $ (3) | $ (3)
(Gains) Losses Reclassified to Interest Expense [Member] |  |  |  |
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] |  |  |  |
Total before tax | $ (1) | $ (1) | (4) | (4)
(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 | $ 1 | $ 1 | 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 |  |  | $ (1) | $ (1)

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