Financial Information by Segment & Geographic Area - Financial Information by Reportable Segment (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Revenues from external customers	$ 659	$ 518	$ 710	$ 727	$ 827	$ 845	$ 844	$ 821	$ 2,614	$ 3,337	$ 3,242
Depreciation and amortization									158	148	149
Equity in earnings of affiliated companies									3	1	2
Income (loss) from continuing operations before income taxes									(33)	255	117
Assets	2,781				3,004				2,781	3,004	3,244
Total expenditures for additions to long-lived assets									169	247	229
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues from external customers									2,442	3,144	3,126
Depreciation and amortization									156	147	152
Equity in earnings of affiliated companies									4	3	7
Income (loss) from continuing operations before income taxes									283	444	480
Assets	2,518				2,637				2,518	2,637	2,876
Total expenditures for additions to long-lived assets									166	242	224
Unallocated and Other [Member]
Segment Reporting Information [Line Items]
Revenues from external customers									172	193	116
Depreciation and amortization									2	1	(3)
Equity in earnings of affiliated companies									(1)	(2)	(5)
Income (loss) from continuing operations before income taxes									(316)	(189)	(363)
Assets	263				367				263	367	368
Total expenditures for additions to long-lived assets									3	5	5
Reinforcement Materials [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues from external customers									1,256	1,815	1,774
Depreciation and amortization									68	69	70
Equity in earnings of affiliated companies										(1)	1
Income (loss) from continuing operations before income taxes									162	266	279
Assets	1,077				1,177				1,077	1,177	1,319
Total expenditures for additions to long-lived assets									66	82	97
Performance Chemicals [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues from external customers									933	995	1,028
Depreciation and amortization									64	51	48
Equity in earnings of affiliated companies									1	1
Income (loss) from continuing operations before income taxes									118	152	200
Assets	1,145				1,024				1,145	1,024	919
Total expenditures for additions to long-lived assets									92	148	94
Purification Solutions [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues from external customers									253	278	279
Depreciation and amortization									24	26	32
Equity in earnings of affiliated companies									3	3	6
Income (loss) from continuing operations before income taxes									3	2	(7)
Assets	$ 296				$ 436				296	436	460
Total expenditures for additions to long-lived assets									$ 8	11	16
Specialty Fluids [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues from external customers										56	45
Depreciation and amortization										1	2
Income (loss) from continuing operations before income taxes										24	8
Assets											178
Total expenditures for additions to long-lived assets										$ 1	$ 17