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Subsequent events (Details) - Subsequent Event - Discontinued operations - Management Services $ in Millions | Oct. 12, 2019 USD ($)
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Subsequent Events |
Consideration received | $ 2,405
Contingent consideration, prior work claims | $ 150

X
- Definition Amount of contingent consideration received or receivable attributable to certain claims related to prior work and engagements. + References No definition available. + Details
Name: | acm_DisposalGroupIncludingDiscontinuedOperationContingentConsideration
Namespace Prefix: | acm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of consideration received or receivable for the disposal of assets and liabilities, including discontinued operation. + References No definition available. + Details
Name: | us-gaap_DisposalGroupIncludingDiscontinuedOperationConsideration
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event. + References No definition available. + Details
Name: | us-gaap_SubsequentEventLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DisposalGroupClassificationAxis=us-gaap_SegmentDiscontinuedOperationsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis=acm_ManagementSupportServicesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |