Significant Accounting Policies - Additional Information (Details) $ in Millions			3 Months Ended	9 Months Ended
		Jul. 01, 2018 ReportableSegment	Jun. 30, 2019 USD ($)	Jun. 30, 2019 USD ($) ReportableSegment
Significant Accounting Policies [Line Items]
Number of reportable segments | ReportableSegment		3		2
Recognition of investment gain within net interest and other expense			$ 6	$ 3
Accounting Standards Update 2016-16 [Member] | Maximum [Member]
Significant Accounting Policies [Line Items]
Cumulative-effect adjustment, reclassification from other current assets and deferred taxes to retained earnings			$ 1	1
Accumulated other comprehensive income (loss) [Member]
Significant Accounting Policies [Line Items]
Unrealized gains on equity securities on reclassification from AOCI to retained earnings	[1],[2]			$ 34

[1] At June 30, 2019 and September 30, 2018, the after-tax accumulated other comprehensive loss attributable to Ashland of $367 million and $291 million, respectively, was each comprised of net unrealized translation losses of $365 million and $328 million, respectively, net unrealized gains on investment securities of zero and $34 million, respectively, and unrecognized prior service costs as a result of certain employee benefit plan amendments of $3 million and unrecognized prior services credits of $3 million, respectively.

[2] Represents the cumulative-effect adjustment related to the adoption of the new guidance related to the accounting for equity securities and the tax effects of intercompany transfers during fiscal 2019. See Note A for more information.