Financial Information by Segment - Financial Information by Reportable Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Segment Reporting Information [Line Items]
Revenues from external customers	$ 842	$ 710	$ 1,588	$ 1,437
Income (loss) before income taxes	118	12	217	62
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	791	664	1,492	1,344
Income (loss) before income taxes	149	95	289	181
Unallocated and Other [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	51	46	96	93
Income (loss) before income taxes	(31)	(83)	(72)	(119)
Reinforcement Materials [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	434	355	809	734
Income (loss) before income taxes	89	61	177	108
Performance Chemicals [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	294	245	561	487
Income (loss) before income taxes	58	31	112	72
Purification Solutions [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	63	64	$ 122	123
Income (loss) before income taxes	$ 2	$ 3		$ 1