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Significant Accounting Policies - Additional Information (Details) $ in Millions |  | 3 Months Ended
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Jul. 01, 2018 ReportableSegment | Dec. 31, 2018 USD ($) ReportableSegment
Significant Accounting Policies [Line Items] |  |  |
Number of reportable segments | ReportableSegment |  | 3 | 2
Unrealized gains on equity securities on reclassification from AOCI to retained earnings | [1] |  | $ 1
Recognition of unrealized loss within net interest and other financing expense |  |  | 30
Accounting Standards Update 2016-16 [Member] |  |  |
Significant Accounting Policies [Line Items] |  |  |
Cumulative-effect adjustment, reclassification from other current assets to retained earnings |  |  | 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] | 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 the three months ended December 31, 2018. See Note A for more information.
[2] | At December 31, 2018 and September 30, 2018, the after-tax accumulated other comprehensive loss attributable to Ashland of $362 million and $291 million, respectively, was each comprised of net unrealized translation losses of $359 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.

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