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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.

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