Financial Information by Segment - Revenue from External Customers by Geographic Region (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Segment Reporting Information [Line Items]
Net sales and other operating revenues	$ 923	$ 1,016	$ 2,814	$ 2,993
Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	893	981	2,720	2,894
Unallocated and Other [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	30	35	94	99
Americas [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	328	368	997	1,077
Asia Pacific [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	331	377	1,059	1,132
Europe, Middle East and Africa [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	234	236	664	685
Reinforcement Materials [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	573	649	1,778	1,966
Reinforcement Materials [Member] | Americas [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	230	265	714	793
Reinforcement Materials [Member] | Asia Pacific [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	199	245	659	760
Reinforcement Materials [Member] | Europe, Middle East and Africa [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	144	139	405	413
Performance Chemicals [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	320	332	942	928
Performance Chemicals [Member] | Americas [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	98	103	283	284
Performance Chemicals [Member] | Asia Pacific [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	132	132	400	372
Performance Chemicals [Member] | Europe, Middle East and Africa [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	$ 90	$ 97	$ 259	$ 272