Financial Information by Segment & Geographic Area - Revenue from External Customers by Geographic Region (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Segment Reporting Information [Line Items]
Net sales and other operating revenues	$ 3,931	$ 4,321	$ 3,409
Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	3,788	4,158	3,251
Unallocated and Other [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	143	163	158
Reinforcement Materials [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues		98	57
Reinforcement Materials [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	2,563	2,673	1,838
Performance Chemicals [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues		16	8
Performance Chemicals [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	1,225	1,388	1,156
Purification Solutions [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues		97	257
Purification Solutions [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues		97	257
Americas [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	1,425	1,534	1,129
Americas [Member] | Reinforcement Materials [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	1,046	1,072	708
Americas [Member] | Performance Chemicals [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	379	419	311
Americas [Member] | Purification Solutions [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues		43	110
Asia Pacific [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	1,486	1,630	1,300
Asia Pacific [Member] | Reinforcement Materials [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	995	1,049	777
Asia Pacific [Member] | Performance Chemicals [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	491	567	489
Asia Pacific [Member] | Purification Solutions [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues		14	34
Europe, Middle East and Africa [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	877	994	822
Europe, Middle East and Africa [Member] | Reinforcement Materials [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	522	552	353
Europe, Middle East and Africa [Member] | Performance Chemicals [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues	$ 355	402	356
Europe, Middle East and Africa [Member] | Purification Solutions [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues		$ 40	$ 113