Financial Information by Segment & Geographic Area - Financial Information by Reportable Segment (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Segment Reporting Information [Line Items]
Revenues	$ 3,994	$ 3,931	$ 4,321
Depreciation and amortization	151	144	146
Income (Loss) from Equity Method Investments	6	5	10
Income (loss) from operations before income taxes	529	451	335
Assets	3,736	3,604	3,525
Total expenditures for additions to long-lived assets	249	265	221
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues	3,860	3,788	4,158
Depreciation and amortization	144	142	145
Income (Loss) from Equity Method Investments	6	5	10
Income (loss) from operations before income taxes	701	607	642
Assets	3,245	3,105	3,149
Total expenditures for additions to long-lived assets	241	257	217
Unallocated and Other [Member]
Segment Reporting Information [Line Items]
Revenues	134	143	163
Depreciation and amortization	7	2	1
Income (loss) from operations before income taxes	(172)	(156)	(307)
Assets	491	499	376
Total expenditures for additions to long-lived assets	8	8	4
Reinforcement Materials [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues	2,610	2,563	2,673
Depreciation and amortization	66	70	70
Income (Loss) from Equity Method Investments	2	2	4
Income (loss) from operations before income taxes	537	482	408
Assets	1,751	1,632	1,691
Total expenditures for additions to long-lived assets	165	149	114
Performance Chemicals [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues	1,250	1,225	1,388
Depreciation and amortization	78	72	72
Income (Loss) from Equity Method Investments	4	3	5
Income (loss) from operations before income taxes	164	125	234
Assets	1,494	1,473	1,458
Total expenditures for additions to long-lived assets	$ 76	$ 108	100
Purification Solutions [Member]
Segment Reporting Information [Line Items]
Revenues			97
Purification Solutions [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues			97
Depreciation and amortization			3
Income (Loss) from Equity Method Investments			1
Total expenditures for additions to long-lived assets			$ 3