Litigation, Claims and Contingencies - Summary of Progression of Insurance Receivable (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended			12 Months Ended
	Mar. 31, 2020		Jun. 30, 2019	Mar. 31, 2019	Mar. 31, 2020		Mar. 31, 2019	Sep. 30, 2019		Sep. 30, 2018	Sep. 30, 2017
Movement in Loss Contingency Receivable, Increase (Decrease) [Roll Forward]
Insurance settlement	$ 3			$ 0	$ 3		$ 1
Ashland [Member]
Movement in Loss Contingency Receivable, Increase (Decrease) [Roll Forward]
Insurance receivable - beginning of year			$ 136		123	[1]	140	$ 140		$ 155	$ 151
Receivable adjustment			(5)		0		0	(5)		(5)	15
Insurance settlement					(10)		0	0		0	(5)
Amounts collected					(4)		(4)	(12)		(10)	(6)
Insurance receivable - end of period	109	[1]		136	109	[1]	136	123	[1]	140	155
Hercules [Member]
Movement in Loss Contingency Receivable, Increase (Decrease) [Roll Forward]
Insurance receivable - beginning of year			54		49		54	54		68	63
Receivable adjustment			$ (5)		0		0	(5)		(14)	5
Insurance receivable - end of period	$ 49			$ 54	$ 49		$ 54	$ 49		$ 54	$ 68

[1] Included $13 million and $15 million classified in accounts receivable on the Condensed Consolidated Balance Sheets as of March 31, 2020 and September 30, 2019, respectively