Financial Information by Segment - Financial Information by Reportable Segment (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]
Revenues from external customers	$ 923	$ 1,016	$ 2,814	$ 2,993
Income (loss) before income taxes	155	158	450	394
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	893	981	2,720	2,894
Income (loss) before income taxes	185	191	541	534
Unallocated and Other [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	30	35	94	99
Income (loss) before income taxes	(30)	(33)	(91)	(140)
Reinforcement Materials [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	573	649	1,778	1,966
Income (loss) before income taxes	128	136	389	414
Performance Chemicals [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	320	332	942	928
Income (loss) before income taxes	$ 57	$ 55	$ 152	$ 120