Financial Information by Segment - Financial Information by Reportable Segment (Detail) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2024	Dec. 31, 2023
Segment Reporting Information [Line Items]
Revenues from external customers	$ 955	$ 958
Income (loss) before income taxes	144	94
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	922	926
Income (loss) before income taxes	175	163
Unallocated and Other [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	33	32
Income (loss) before income taxes	(31)	(69)
Reinforcement Materials [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	611	641
Income (loss) before income taxes	130	129
Performance Chemicals [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	311	285
Income (loss) before income taxes	$ 45	$ 34