Financial Information by Segment & Geographic Area - Revenue from External Customers by Geographic Region (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]
Net sales and other operating revenues	$ 659	$ 518	$ 710	$ 727	$ 827	$ 845	$ 844	$ 821	$ 2,614	$ 3,337	$ 3,242
Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues									2,442	3,144	3,126
Unallocated and Other [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues									172	193	116
Reinforcement Materials [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues									1,256	1,815	1,774
Performance Chemicals [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues									933	995	1,028
Purification Solutions [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues									253	278	279
Specialty Fluids [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues										56	$ 45
Americas [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues									868	1,114
Americas [Member] | Reinforcement Materials [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues									490	688
Americas [Member] | Performance Chemicals [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues									266	294
Americas [Member] | Purification Solutions [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues									112	126
Americas [Member] | Specialty Fluids [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues										6
Asia Pacific [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues									931	1,158
Asia Pacific [Member] | Reinforcement Materials [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues									529	769
Asia Pacific [Member] | Performance Chemicals [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues									368	353
Asia Pacific [Member] | Purification Solutions [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues									34	35
Asia Pacific [Member] | Specialty Fluids [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues										1
Europe, Middle East and Africa [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues									643	872
Europe, Middle East and Africa [Member] | Reinforcement Materials [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues									237	358
Europe, Middle East and Africa [Member] | Performance Chemicals [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues									299	348
Europe, Middle East and Africa [Member] | Purification Solutions [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues									$ 107	117
Europe, Middle East and Africa [Member] | Specialty Fluids [Member]
Segment Reporting Information [Line Items]
Net sales and other operating revenues										$ 49