Financial Information by Segment - Financial Information by Reportable Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2025	Mar. 31, 2024	Mar. 31, 2025	Mar. 31, 2024
Segment Reporting Information [Line Items]
Revenues from external customers	$ 936	$ 1,019	$ 1,891	$ 1,977
Income (loss) before income taxes	151	142	295	236
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	905	987	1,827	1,913
Income (loss) before income taxes	181	180	356	343
Unallocated and Other [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	31	32	64	64
Income (loss) before income taxes	(30)	(38)	(61)	(107)
Reinforcement Materials [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	594	676	1,205	1,317
Income (loss) before income taxes	131	149	261	278
Performance Chemicals [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	311	311	622	596
Income (loss) before income taxes	$ 50	$ 31	$ 95	$ 65