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Business Acquisitions, Goodwill and Intangible Assets (Details) $ in Millions | 12 Months Ended
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Sep. 30, 2018 USD ($) item
Business Acquisitions, Goodwill and Intangible Assets |
Number of acquisitions | item | 1
Total consideration | $ | $ 5.6

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

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- Definition The number of businesses acquired by the entity during the period. + References No definition available. + Details
Name: | us-gaap_NumberOfBusinessesAcquired
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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- Definition The cash outflow associated with the acquisition of business during the period. The cash portion only of the acquisition price. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 13 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3213-108585 + Details
Name: | us-gaap_PaymentsToAcquireBusinessesGross
Namespace Prefix: | us-gaap_
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