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New Accounting Pronouncements and Changes in Accounting (Details) - Accounting Standards Update April 2015 - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2016 | Sep. 30, 2016
Recent Accounting Pronouncements |  |
Unamortized debt issuance costs (noncurrent assets) | $ (54.0) | $ (56.8)
Unamortized debt issuance costs (long term debt) | $ 54.0 | $ 56.8

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- Definition Represents unamortized debt issued costs currently classified as other current assets. + References No definition available. + Details
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Namespace Prefix: | acm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Represents information pertaining to unamortized debt issuance costs classified as long-term debt. + References No definition available. + Details
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Namespace Prefix: | acm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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