Fair Value Measurements - Additional Information (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended		12 Months Ended
		Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Sep. 30, 2018
Restricted Investment [Abstract]
Unrealized gains on equity securities	[1]	$ 6	$ 4	$ 3	$ 6
Fair Value Disclosures [Abstract]
Gains (loss) related to deferred compensation investments		2	$ 2	3	$ 6
Other financial instruments [Abstract]
Long-term Debt, Carrying Value		2,299		2,299		$ 2,307
Long-term Debt, Fair Value		$ 2,437		$ 2,437		2,372
Accounting Standards Update 2016-01 | Reclassified from AOCI to Retained Earnings [Member]
Restricted Investment [Abstract]
Unrealized gains on equity securities						54
Deferred income taxes						$ 20

[1] 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 year 2019, the net unrealized losses during the three and nine months ended June 30, 2019 were recorded within the net interest and other expense (income) caption in the Statements of Consolidated Comprehensive Income (Loss). In 2018 these net unrealized gains (losses) were in AOCI.