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Significant Accounting Policies - Pension Plans (Details)	12 Months Ended
	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%

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- 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

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- 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

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- References

No definition available.

+ Details

Name:	us-gaap_AccountingPoliciesAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration