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Significant Accounting Policies - Additional Information (Detail) - USD ($)	3 Months Ended		12 Months Ended
	Jun. 30, 2024	Mar. 31, 2024	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Significant Accounting Policies [Line Items]
Equity affiliate investments			$ 23,000,000	$ 20,000,000
Capitalized interest			9,000,000	6,000,000	$ 3,000,000
Asset retirement obligation reserve			11,000,000	11,000,000
Changes in allowance			$ 0	0	0
Chance of utilizing the associated benefit for valuation allowances, maximum			50.00%
Equity Method Investments [Member]
Significant Accounting Policies [Line Items]
Dividends declared			$ 2,000,000	2,000,000	1,000,000
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]
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)			$ (46,000,000)	(35,000,000)	(13,000,000)
Investment income (loss)			17,000,000	16,000,000	5,000,000
Other Nonoperating Income (Expense) [Member] | Revaluation of Non-functional Currency [Member]
Significant Accounting Policies [Line Items]
Foreign currency transaction gain (loss)			(43,000,000)	(33,000,000)	(15,000,000)
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)	$ 0
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

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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.

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Significant accounting policies.

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