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Income Taxes - Additional Information (Detail) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2023 | Sep. 30, 2022
Income Taxes [Line Items] |  |  |  |
Increased/(decreased) valuation in allowance | $ 31 |  |  |
Other tax credit carryforwards | 140 |  |  |
Undistributed earnings | 2,000 |  |  |
Unrecognized tax benefits | 29 | $ 22 | $ 20 | $ 159
Unrecognized tax benefits, offset against deferred tax assets | 11 | 13 |  |
Non cash income tax expense (benefit) |  | 24 |  |
Accruals for penalties and interest | 13 | 10 |  |
Total penalties and interest | 5 | 5 | 2 |
Favorable impact on tax provision | 29 |  |  |
Purification Solutions [Member] |  |  |  |
Income Taxes [Line Items] |  |  |  |
Income tax benefit from tax loss related to divestiture |  |  | $ 137 | $ 137
Other Liabilities [Member] |  |  |  |
Income Taxes [Line Items] |  |  |  |
Unrecognized tax benefits, recorded | 9 | 9 |  |
Accounts payable and accrued liabilities [Member] |  |  |  |
Income Taxes [Line Items] |  |  |  |
Unrecognized tax benefits, recorded | 9 |  |  |
Maximum [Member] |  |  |  |
Income Taxes [Line Items] |  |  |  |
Other tax credit carryforwards | $ 1 |  |  |
Maximum [Member] | United States Internal Revenue Service (IRS) [Member] |  |  |  |
Income Taxes [Line Items] |  |  |  |
Tax years remain subject to examination | 2024 |  |  |
Minimum [Member] | United States Internal Revenue Service (IRS) [Member] |  |  |  |
Income Taxes [Line Items] |  |  |  |
Tax years remain subject to examination | 2022 |  |  |
Foreign Jurisdictions [Member] |  |  |  |
Income Taxes [Line Items] |  |  |  |
NOLs | $ 30 |  |  |
Foreign Jurisdictions [Member] | Maximum [Member] |  |  |  |
Income Taxes [Line Items] |  |  |  |
Tax years remain subject to examination | 2024 |  |  |
Foreign Jurisdictions [Member] | Minimum [Member] |  |  |  |
Income Taxes [Line Items] |  |  |  |
Tax years remain subject to examination | 2006 |  |  |
State Tax Authorities [Member] | Maximum [Member] |  |  |  |
Income Taxes [Line Items] |  |  |  |
Tax years remain subject to examination | 2024 |  |  |
State Tax Authorities [Member] | Minimum [Member] |  |  |  |
Income Taxes [Line Items] |  |  |  |
Tax years remain subject to examination | 2019 |  |  |
U.S. Defined Benefit Plans [Member] |  |  |  |
Income Taxes [Line Items] |  |  |  |
Increased/(decreased) valuation in allowance | $ 516 |  |  |
U.S. Defined Benefit Plans [Member] | Maximum [Member] |  |  |  |
Income Taxes [Line Items] |  |  |  |
Increased/(decreased) valuation in allowance |  | 494 |  |
U.S. Defined Benefit Plans [Member] | Minimum [Member] |  |  |  |
Income Taxes [Line Items] |  |  |  |
Increased/(decreased) valuation in allowance |  | $ 22 |  |

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