Fair Value Measurements - Summary of Investment Income, Net Gains and Losses Realized and Disbursements Related to Investments (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended			12 Months Ended
		Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020		Mar. 31, 2019	Sep. 30, 2019
Fair Value Disclosures [Abstract]
Investment income		$ 3	$ 2	$ 7	[1]	$ 5	$ 10	[1]
Net gains (losses) unrealized	[2]	(32)	27	(23)		(3)
Disbursements		$ (9)	$ (12)	$ (19)		$ (20)	$ (32)

[1] Investment income for the demand deposit includes interest income as well as dividend income transferred from the equity and corporate bond mutual funds.

[2] Ashland determined that all unrealized gains and losses were related to equity securities with readily determinable fair values. Due to the new accounting guidance adopted in the first quarter of fiscal 2019, the net unrealized gains (losses) during the three months ended December 31, 2018 and forward were recorded within the net interest and other expense caption in the Statements of Consolidated Comprehensive Income (Loss).