Financial Information by Segment & Geographic Area - Schedule of Income (Loss) from Continuing Operations before Income Taxes and Equity in Earnings of Affiliated Companies for Unallocated and Other (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Interest expense		$ (53)	$ (59)	$ (54)
Global restructuring activities		(19)	(16)	30
Gains (losses) on sale of investments				10
Purification Solutions [Member]
Segment Reporting Information [Line Items]
Goodwill and long-lived assets impairment charge	$ (224)
Unallocated and Other [Member]
Segment Reporting Information [Line Items]
Interest expense		(53)	(59)	(54)
Unallocated corporate costs		(41)	(50)	(61)
General unallocated income (expense)		(1)	8	2
Less: Equity in earnings of affiliated companies, net of tax		3	1	2
Income from continuing operations before income taxes and equity in earnings of affiliated companies		(316)	(189)	(363)
Certain Items [Member]
Segment Reporting Information [Line Items]
Legal and environmental matters and reserves		(54)	(21)	(16)
Global restructuring activities		(19)	(16)	30
Employee benefit plan settlement and other charges		(10)	1
Acquisition and integration-related charges		(5)	(6)	(2)
Inventory reserve adjustment		(2)		(13)
Executive transition costs			(1)	(2)
Indirect tax settlement credits		3
Gains (losses) on sale of investments				10
Other certain items		(1)	(4)	(1)
Total certain items, pre-tax		(218)	(87)	(248)
Certain Items [Member] | Marshall Mine [Member]
Segment Reporting Information [Line Items]
Loss on sale and asset impairment charge		(129)
Certain Items [Member] | Specialty Fluids [Member]
Segment Reporting Information [Line Items]
Loss on sale and asset impairment charge		$ (1)	(29)
Certain Items [Member] | Equity Affiliate Investment [Member]
Segment Reporting Information [Line Items]
Investment impairment charge			$ (11)
Certain Items [Member] | Purification Solutions [Member]
Segment Reporting Information [Line Items]
Goodwill and long-lived assets impairment charge				$ (254)