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Condensed Consolidating Financial Information - Debt (Details)	6 Months Ended
	Mar. 31, 2018
Condensed Consolidating Financial Information
Ownership of directly and indirectly owned subsidiaries (as a percent)	100.00%

X

- References

No definition available.

+ Details

Name:	acm_CondensedConsolidatingFinancialInformation
Namespace Prefix:	acm_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Percentage of subsidiary's or equity method investee's stock owned by parent immediately after all stock transactions.

+ References

No definition available.

+ Details

Name:	us-gaap_SubsidiaryOrEquityMethodInvesteeCumulativePercentageOwnershipAfterAllTransactions
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration