Divestitures - Additional Information (Detail) - USD ($) $ in Millions		3 Months Ended			12 Months Ended
	Sep. 30, 2020	Sep. 30, 2020	Mar. 31, 2020	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Specialty Fluids [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Impairment charge					$ 1	$ 29
Marshall Mine [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Impairment charge		$ 129			$ 129
Sinomine (Hong Kong) Rare Metals Resources Co. Limited [Member] | Specialty Fluids [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Pre-tax loss on sale			$ 1			$ 9
Impairment charge				$ 20
ADA Carbon Solutions, LLC [Member] | Marshall Mine [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Pre-tax loss on sale		67
Impairment charge		$ 62
Closure costs	$ 9
Closure costs payment period					5 years