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Debt - Summary of Net Interest and Other Expense (Income) (Parenthetical) (Details) - USD ($) $ in Millions | 12 Months Ended
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Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2017
Debt Instrument [Line Items] |  |  |  |
Accelerated amortization | [1] | $ 6 | $ 1 | $ 101
Notes due 2018 and 2022 [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Loss on early retirement of debt |  |  |  | 16
Gain (loss) on repurchase of debt instrument |  |  |  | (3)
Junior Subordinated Debt [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Gain (loss) on repurchase of debt instrument |  |  |  | $ 5
[1] | Fiscal year 2017 includes $92 million of accelerated accretion of the recorded debt discount for the 2029 Notes, while the remaining amounts in each year related to the accelerated amortization of debt issuance costs.

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- Definition The portion of the component of interest expense representing the noncash expenses charged against earnings in the period to allocate debt discount and premium, and the costs to issue debt and obtain financing over the related debt instruments for amount utilizing accelerated amortization periods generally due to termination, prepayment or refinancing. + References No definition available. + Details
Name: | ash_AmortizationOfFinancingCostsAndDiscountsAcceleratedAmortization
Namespace Prefix: | ash_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Charges incurred for the early repayment of debt, including but not limited to the accelerated amortization for deferred debt issuance costs, accelerated amortization for the original issue discount, or premiums paid as a result of the prepayment of the debt. + References No definition available. + Details
Name: | ash_ChargesIncurredForEarlyRepaymentOfDebt
Namespace Prefix: | ash_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of gain (loss) from the difference between the repurchase price of a debt instrument initially issued by the entity and the net carrying amount of the debt at the time of its repurchase. + References No definition available. + Details
Name: | us-gaap_GainLossOnRepurchaseOfDebtInstrument
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_DebtInstrumentAxis=us-gaap_JuniorSubordinatedDebtMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |