Financial Information by Segment - Financial Information by Reportable Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023
Segment Reporting Information [Line Items]
Revenues from external customers	$ 1,019	$ 1,033	$ 1,977	$ 1,998
Income (loss) before income taxes	142	110	236	194
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	987	998	1,913	1,927
Income (loss) before income taxes	180	150	343	273
Unallocated and Other [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	32	35	64	71
Income (loss) before income taxes	(38)	(40)	(107)	(79)
Reinforcement Materials [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	676	672	1,317	1,315
Income (loss) before income taxes	149	122	278	216
Performance Chemicals [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	311	326	596	612
Income (loss) before income taxes	$ 31	$ 28	$ 65	$ 57