Fair Value Measurements - Summary of Investment Income, Net Unrealized Gains and Losses, Realized Gains and Disbursements Related to Investments (Details) - USD ($) $ in Millions		12 Months Ended
		Sep. 30, 2021		Sep. 30, 2020		Sep. 30, 2019
Fair Value Disclosures [Abstract]
Investment income		$ 12	[1]	$ 10	[1]	$ 10
Net gains (losses)	[2]	21		20		7
Disbursements		$ (33)		$ (35)		$ (32)

[1] Investment income relates to the demand deposit and includes interest income as well as dividend income transferred from the equity and fixed income mutual funds.

[2] Ashland determined that all gains and (losses) were related to investment securities with readily determinable fair values. Due to the new accounting guidance adopted in the first quarter of fiscal year 2019, the net unrealized gains and (losses) during the year ended September 30, 2019 and forward were recorded within the net interest and other expense caption in the Statements of Consolidated Income (Loss). In 2018 these net unrealized gains (losses) were in AOCI.