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Fair Value Measurements - Summary of Financial Instruments Subject to Recurring Fair Value Measurements (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Millions | Jun. 30, 2020 | Sep. 30, 2019
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Carrying (Reported) Amount, Fair Value Disclosure [Member] |  |  |  |  |
ASSETS |  |  |  |  |
Cash and cash equivalents |  | $ 416 |  | $ 232 |
Restricted investments |  | 327 | [1] | 334 | [2]
Investment of captive insurance company | [3] | 7 |  | 5 |
Foreign currency derivatives |  | 1 |  | 2 |
Total assets at fair value |  | 751 |  | 573 |
Liabilities |  |  |  |  |
Foreign currency derivatives |  | 2 |  | 2 |
Estimate of Fair Value, Fair Value Disclosure [Member] |  |  |  |  |
ASSETS |  |  |  |  |
Cash and cash equivalents |  | 416 |  | 232 |
Restricted investments |  | 327 | [1] | 334 | [2]
Investment of captive insurance company | [3] | 7 |  | 5 |
Foreign currency derivatives |  | 1 |  | 2 |
Total assets at fair value |  | 751 |  | 573 |
Liabilities |  |  |  |  |
Foreign currency derivatives |  | 2 |  | 2 |
Fair Value, Inputs, Level 1 [Member] |  |  |  |  |
ASSETS |  |  |  |  |
Cash and cash equivalents |  | 416 |  | 232 |
Restricted investments |  | 327 | [1] | 334 | [2]
Investment of captive insurance company | [3] | 7 |  | 5 |
Foreign currency derivatives |  | 0 |  | 0 |
Total assets at fair value |  | 750 |  | 571 |
Liabilities |  |  |  |  |
Foreign currency derivatives |  | 0 |  | 0 |
Fair Value, Inputs, Level 2 [Member] |  |  |  |  |
ASSETS |  |  |  |  |
Cash and cash equivalents |  | 0 |  | 0 |
Restricted investments |  | 0 | [1] | 0 | [2]
Investment of captive insurance company | [3] | 0 |  | 0 |
Foreign currency derivatives |  | 1 |  | 2 |
Total assets at fair value |  | 1 |  | 2 |
Liabilities |  |  |  |  |
Foreign currency derivatives |  | 2 |  | 2 |
Fair Value, Inputs, Level 3 [Member] |  |  |  |  |
ASSETS |  |  |  |  |
Cash and cash equivalents |  | 0 |  | 0 |
Restricted investments |  | 0 | [1] | 0 | [2]
Investment of captive insurance company | [3] | 0 |  | 0 |
Foreign currency derivatives |  | 0 |  | 0 |
Total assets at fair value |  | 0 |  | 0 |
Liabilities |  |  |  |  |
Foreign currency derivatives |  | $ 0 |  | $ 0 |
[1] | Included in restricted investments is $30 million classified in the other current assets caption on the Condensed Consolidated Balance Sheets.
[2] | Included in restricted investments is $24 million classified in the other current assets caption on the Condensed Consolidated Balance Sheets.
[3] | Included in other noncurrent assets in the Condensed Consolidated Balance Sheets.

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- Definition This element represents the fair value of investments of a captive insurance company. + References No definition available. + Details
Name: | ash_InvestmentsOfCaptiveInsuranceCompanyFairValueDisclosure
Namespace Prefix: | ash_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_AssetsAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 + Details
Name: | us-gaap_AssetsFairValueDisclosure
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. + References No definition available. + Details
Name: | us-gaap_CashAndCashEquivalentsFairValueDisclosure
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Fair value portion of asset contracts related to the exchange of different currencies, including, but not limited to, foreign currency options, forward contracts, and swaps. + References No definition available. + Details
Name: | us-gaap_ForeignCurrencyContractAssetFairValueDisclosure
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Fair value portion of liability contracts related to the exchange of different currencies, including, but not limited to, foreign currency options, forward (delivery or nondelivery) contracts, and swaps entered into. + References No definition available. + Details
Name: | us-gaap_ForeignCurrencyContractsLiabilityFairValueDisclosure
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_LiabilitiesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Cash and investments whose use in whole or in part is restricted for the long-term, generally by contractual agreements or regulatory requirements. For use in an unclassified balance sheet. + References No definition available. + Details
Name: | us-gaap_RestrictedCashAndInvestments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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