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Financial Information by Segment & Geographic Area - Schedule of Details of Segment EBIT Including Segment Revenue and Significant Segment Expenses (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Segment Reporting Information [Line Items]
Segment revenues from external customers	$ 3,713	$ 3,994	$ 3,931
Segment cost of sales	2,773	3,034	3,092
Operating Segments [Member]
Segment Reporting Information [Line Items]
Segment revenues from external customers	3,591	3,860	3,788
Segment cost of sales	(2,625)	(2,874)	(2,933)
Segment operating expenses	(271)	(291)	(253)
Other Segment Items	7	6	5
Segment EBIT	702	701	607
Reinforcement Materials [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Segment revenues from external customers	2,341	2,610	2,563
Segment cost of sales	(1,704)	(1,927)	(1,959)
Segment operating expenses	(132)	(148)	(124)
Other Segment Items	3	2	2
Segment EBIT	508	537	482
Performance Chemicals [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Segment revenues from external customers	1,250	1,250	1,225
Segment cost of sales	(921)	(947)	(974)
Segment operating expenses	(139)	(143)	(129)
Other Segment Items	4	4	3
Segment EBIT	$ 194	$ 164	$ 125

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