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Income Taxes - Summary of Changes in Unrecognized Tax Benefits (Details) - USD ($) $ in Millions | 12 Months Ended
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Sep. 30, 2019 | Sep. 30, 2018
Balance at beginning of period | [1] | $ 164 | $ 194 |
Increases related to positions taken on items from prior years |  | 1 | 5 |
Decreases related to positions taken on items from prior years |  | (5) | (40) |
Increases related to positions taken in the current year |  | 21 | 14 |
Lapse of statute of limitations |  | (10) | (5) |
Settlement of uncertain tax positions with tax authorities |  |  | (4) |
Balance at end of period |  | 165 | $ 164 | [1]
Composites/Marl facility [Member] |  |  |  |
Disposition of Composites and Marl facility |  | $ (6) |  |
[1] | Ashland has indemnity receivables from Valvoline and Pharmachem for $32 million and $39 million of the gross unrecognized tax benefits at September 30, 2019 and 2018, respectively.

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- Definition Amount of decrease in unrecognized tax benefits resulting from settlements with taxing authorities. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=84230637&loc=SL6600010-109319 + Details
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- Definition Amount of decrease in unrecognized tax benefits resulting from lapses of applicable statutes of limitations. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a)(4) -URI http://asc.fasb.org/extlink&oid=84230637&loc=SL6600010-109319 + Details
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