Financial Information by Segment - Financial Information by Reportable Segment (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Segment Reporting Information [Line Items]
Revenues from external customers	$ 982	$ 923	$ 2,735	$ 2,814
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	950	893	2,642	2,720
Unallocated and Other [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	32	30	93	94
Reinforcement Materials [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	599	573	1,663	1,778
Performance Chemicals [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	$ 351	$ 320	$ 979	$ 942

Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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+ Details

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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+ Details