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Stock Incentive Plans - Sumamry of Performance Shares/units Granted (Details) - Performance Shares [Member] - $ / shares shares in Millions | 12 Months Ended
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Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Vesting period, start date |  | Oct. 01, 2018 | Oct. 01, 2017 | Oct. 01, 2016
Vesting period, end date |  | Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2019
Target shares granted | [1] | 78 | 101 | 56
Weighted-average fair value per share | [1] | $ 84.93 | $ 68.93 | $ 103.72
[1] | At the end of the performance period, the actual number of shares/units awarded can range from zero to 200% of the target shares/units granted, which is assumed to be 100%. Both the shares granted and weighted-average fair value per share/unit are as of the grant date and have not been adjusted for the Valvoline separation that occurred during 2017.

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- Definition Share based compensation arrangement by share based payment award award vesting end date. + References No definition available. + Details
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- Definition Share based compensation arrangement by share based payment award award vesting start date. + References No definition available. + Details
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Namespace Prefix: | ash_
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- Definition The target number of shares that the company expects to award in the performance based share program. + References No definition available. + Details
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Period Type: | duration

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- Definition The weighted-average fair value per share of the performance shares/units grated for all plans that award Ashland Common Stock. + References No definition available. + Details
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Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | duration

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Details
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