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