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Significant Accounting Policies - Additional Information (Detail) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2024 USD ($) | Mar. 31, 2024 USD ($) | Sep. 30, 2025 USD ($) Customer $ / shares shares | Sep. 30, 2024 USD ($) Customer $ / shares shares | Sep. 30, 2023 USD ($) Customer
Significant Accounting Policies [Line Items] |  |  |  |  |
Equity affiliate investments |  |  | $ 16,000,000 | $ 23,000,000 |
Impairment charge |  |  | 0 | 0 |
Capitalized interest |  |  | 12,000,000 | 9,000,000 | $ 6,000,000
Asset retirement obligation reserve |  |  | $ 18,000,000 | 11,000,000 |
Performance obligation optional exemption |  |  | The Company’s contracts with customers are generally for sales of operating segment products only, do not include other performance obligations, and have an original duration of less than one year. As performance obligations are satisfied in less than a year, the Company has applied the optional exemption of not disclosing remaining performance obligations associated with unfulfilled orders. |  |
Changes in allowance |  |  | $ 0 | $ 0 | 0
Common stock, authorized shares | shares |  |  | 200,000,000 | 200,000,000 |
Common stock, par value | $ / shares |  |  | $ 1 | $ 1 |
Common stock, outstanding shares | shares |  |  | 52,842,481 | 54,297,251 |
Common stock, voting rights |  |  | one vote per share |  |
Chance of utilizing the associated benefit for valuation allowances, maximum |  |  | 50.00% |  |
Contract with customer, asset |  |  | $ 0 |  |
Contract with customer, liability |  |  | 0 |  |
Equity Method Investments [Member] |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Dividends declared |  |  | $ 13,000,000 | $ 2,000,000 | 2,000,000
Minimum [Member] |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Payment terms |  |  | 0 days |  |
Minimum [Member] | Buildings [Member] |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Useful life of property, plant and equipment |  |  | 20 years |  |
Minimum [Member] | Machinery and Equipment [Member] |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Useful life of property, plant and equipment |  |  | 10 years |  |
Minimum [Member] | Other Fixed Assets [Member] |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Useful life of property, plant and equipment |  |  | 3 years |  |
Minimum [Member] | Cabot [Member] | Equity Affiliate Investments [Member] |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Equity method investment, ownership percentage |  |  | 20.00% |  |
Maximum [Member] |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Performance obligation optional exemption remaining duration |  |  | 1 year |  |
Payment terms |  |  | 90 days |  |
Maximum [Member] | Buildings [Member] |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Useful life of property, plant and equipment |  |  | 25 years |  |
Maximum [Member] | Machinery and Equipment [Member] |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Useful life of property, plant and equipment |  |  | 25 years |  |
Maximum [Member] | Other Fixed Assets [Member] |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Useful life of property, plant and equipment |  |  | 25 years |  |
Maximum [Member] | Cabot [Member] | Equity Affiliate Investments [Member] |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Equity method investment, ownership percentage |  |  | 50.00% |  |
Other Nonoperating Income (Expense) [Member] |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Foreign currency transaction gain (loss) |  |  | $ (8,000,000) | (46,000,000) | (35,000,000)
Investment income (loss) |  |  | 2,000,000 | 17,000,000 | 16,000,000
Other Nonoperating Income (Expense) [Member] | Revaluation of Non-functional Currency [Member] |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Foreign currency transaction gain (loss) |  |  | $ (9,000,000) | (43,000,000) | (33,000,000)
Other Nonoperating Income (Expense) [Member] | Single Devaluation Action [Member] |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Foreign currency transaction gain (loss) |  |  |  | $ (33,000,000) | $ 7,000,000
BOPREAL Bonds [Member] |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Foreign currency transaction gain (loss) | $ (2,000,000) |  |  |  |
Investment income (loss) |  | $ (8,000,000) |  |  |
Purchase of bonds to settle foreign payables | $ 8,000,000 | 30,000,000 |  |  |
Proceeds from sale of bonds |  | $ 22,000,000 |  |  |
Revenue Benchmark [Member] | Customer Concentration Risk [Member] |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Number of customer accounted for more than 10 percent | Customer |  |  | 0 | 0 | 0

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- Definition Valuation allowances are provided against deferred tax assets where management believes that the Company has a 50% or less chance of utilizing the associated benefit in the future. + References No definition available. + Details
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