Financial Information by Segment & Geographic Area - Revenue from External Customers by Geographic Region (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Segment Reporting Information [Line Items]
Net sales and other operating revenues	$ 3,713	$ 3,994	$ 3,931
Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	3,591	3,860	3,788
Unallocated and Other [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	122	134	143
Americas [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	1,317	1,434	1,425
Asia Pacific [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	1,401	1,521	1,486
Europe, Middle East and Africa [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	873	905	877
Reinforcement Materials [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	2,341	2,610	2,563
Reinforcement Materials [Member] | Americas [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	944	1,056	1,046
Reinforcement Materials [Member] | Asia Pacific [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	865	1,015	995
Reinforcement Materials [Member] | Europe, Middle East and Africa [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	532	539	522
Performance Chemicals [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	1,250	1,250	1,225
Performance Chemicals [Member] | Americas [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	373	378	379
Performance Chemicals [Member] | Asia Pacific [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	536	506	491
Performance Chemicals [Member] | Europe, Middle East and Africa [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	$ 341	$ 366	$ 355