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Debt and Other Financing Activities - U.S. Accounts Receivable Sales Program - Additional Information (Details) - U.S. Accounts Receivable Sales Program [Member] - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | ---
Mar. 31, 2025 | Mar. 31, 2024 | Mar. 31, 2025 | Mar. 31, 2024 | Sep. 30, 2024
Disclosure Of Sales Program Of Receivable [Line Items] |  |  |  |  |
Loss on sale of receivables | $ 1 | $ 1 | $ 2 | $ 2 |
Sales against the buyers limit |  |  | 60 |  | $ 71
Maximum limit of receivables can transfer |  |  | 60 |  | 71
Amount transferred in accounts receivable |  |  | 73 |  | 85
Gross cash proceeds received for accounts receivable transferred |  |  | 189 | 174 |
Cash proceeds collected |  |  | 200 | 159 |
Increase (decrease) in account receivable sales volume |  |  | 11 | $ 15 |
Maximum [Member] |  |  |  |  |
Disclosure Of Sales Program Of Receivable [Line Items] |  |  |  |  |
Recorded liabilities related to service obligations and limited guarantee | $ 1 |  | $ 1 |  | $ 1

X
- Definition Amount transferred in accounts receivables. + References No definition available. + Details
Name: | ash_AmountTransferredInAccountsReceivables
Namespace Prefix: | ash_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Cash proceeds received for receivables + References No definition available. + Details
Name: | ash_CashProceedsReceivedForReceivables
Namespace Prefix: | ash_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Disclosure of sales program of receivable. + References No definition available. + Details
Name: | ash_DisclosureOfSalesProgramOfReceivableLineItems
Namespace Prefix: | ash_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Gross cash proceeds received for accounts receivables transferred. + References No definition available. + Details
Name: | ash_GrossCashProceedsReceivedForAccountsReceivablesTransferred
Namespace Prefix: | ash_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Increase (decrease) in account receivable sales volume. + References No definition available. + Details
Name: | ash_IncreaseDecreaseInAccountReceivableSalesVolume
Namespace Prefix: | ash_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Maximum limit of receivables for sale. + References No definition available. + Details
Name: | ash_MaximumLimitOfReceivablesForSale
Namespace Prefix: | ash_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Recorded liabilities related to service obligations and limited guarantee. + References No definition available. + Details
Name: | ash_RecordedLiabilitiesRelatedToServiceObligationsAndLimitedGuarantee
Namespace Prefix: | ash_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Sales against the buyers limit + References No definition available. + Details
Name: | ash_SalesAgainstTheBuyersLimit
Namespace Prefix: | ash_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of gain (loss) on sale of accounts receivable. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 860 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Publisher FASB -URI https://asc.fasb.org/1943274/2147481326/860-20-50-5 + Details
Name: | us-gaap_GainLossOnSaleOfAccountsReceivable
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Details
Name: | us-gaap_LongtermDebtTypeAxis=ash_USAccountsReceivableSalesProgramMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |