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v3.10.0.1

Significant Accounting Policies - Pension Plans (Details) | 12 Months Ended
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Sep. 30, 2018
Significant Accounting Policies |
Market-related valuation calculation period (in years) | 5 years
Net unrecognized gain (loss) threshold subject to amortization (in percent) | 10.00%

X
- Definition Represents the percentage of the greater of the projected benefit obligation or the market related value of plan assets that has to be exceeded for amortization of cumulative net unrecognized gains or losses. + References No definition available. + Details
Name: | acm_DefinedBenefitPensionPlanAmortizationOfGainsLossesSpecifiedPercentageToBeExceeded
Namespace Prefix: | acm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the amortization period of the difference between the actual return (including capital, dividends, and interest) and the expected return in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
Name: | acm_DefinedBenefitPensionPlanAmortizationPeriodOfDifferenceBetweenActualReturnAndExpectedReturn
Namespace Prefix: | acm_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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