Financial Information by Segment & Geographic Area - Financial Information by Reportable Segment (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Segment Reporting Information [Line Items]
Revenues	$ 3,931	$ 4,321	$ 3,409
Depreciation and amortization	144	146	160
Income (Loss) from Equity Method Investments	5	10	3
Income (loss) from operations before income taxes	451	335	406
Assets	3,604	3,525	3,306
Total expenditures for additions to long-lived assets	265	221	198
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues	3,788	4,158	3,251
Depreciation and amortization	142	145	159
Income (Loss) from Equity Method Investments	5	10	4
Income (loss) from operations before income taxes	607	642	550
Assets	3,105	3,149	3,029
Total expenditures for additions to long-lived assets	257	217	193
Unallocated and Other [Member]
Segment Reporting Information [Line Items]
Revenues	143	163	158
Depreciation and amortization	2	1	1
Income (Loss) from Equity Method Investments			(1)
Income (loss) from operations before income taxes	(156)	(307)	(144)
Assets	499	376	277
Total expenditures for additions to long-lived assets	8	4	5
Reinforcement Materials [Member]
Segment Reporting Information [Line Items]
Revenues		98	57
Reinforcement Materials [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues	2,563	2,673	1,838
Depreciation and amortization	70	70	70
Income (Loss) from Equity Method Investments	2	4
Income (loss) from operations before income taxes	482	408	329
Assets	1,632	1,691	1,421
Total expenditures for additions to long-lived assets	149	114	104
Performance Chemicals [Member]
Segment Reporting Information [Line Items]
Revenues		16	8
Performance Chemicals [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues	1,225	1,388	1,156
Depreciation and amortization	72	72	73
Income (Loss) from Equity Method Investments	3	5	2
Income (loss) from operations before income taxes	125	234	211
Assets	1,473	1,458	1,325
Total expenditures for additions to long-lived assets	$ 108	100	80
Purification Solutions [Member]
Segment Reporting Information [Line Items]
Revenues		97	257
Purification Solutions [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues		97	257
Depreciation and amortization		3	16
Income (Loss) from Equity Method Investments		1	2
Income (loss) from operations before income taxes			10
Assets			283
Total expenditures for additions to long-lived assets		$ 3	$ 9