This element represents loss contingency amount estimated recovery from third party

No definition available.

Name:	acm_LossContingencyAmountEstimatedRecoveryFromThirdParty
Namespace Prefix:	acm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Surety bonds issued primarily to support the Company's project execution and insurance program commitments.

+ References

No definition available.

+ Details

Name:	acm_LossContingencySuretyBondsIssued
Namespace Prefix:	acm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the estimated minimum cost to complete the project and promised amenities.

+ References

No definition available.

+ Details

Name:	acm_MinimumEstimatedCostToCompleteProjectAndPromisedAmenities
Namespace Prefix:	acm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the number of affiliates for which inspections occurred on accounts and records on those affiliates.

+ References

No definition available.

+ Details

Name:	acm_NumberOfAffiliatesForWhichInspectionsOccurredOnAccountsAndRecords
Namespace Prefix:	acm_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

- Definition

Reflects the estimated amount of loss from the specified contingency as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14435-108349

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 9
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14557-108349

+ Details

Name:	us-gaap_LossContingencyEstimateOfPossibleLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The estimated maximum amount of possible loss from the contingency.

+ References

+ Details

Name:	us-gaap_LossContingencyRangeOfPossibleLossMaximum
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The estimated minimum amount of possible loss from the contingency.

+ References

+ Details

Name:	us-gaap_LossContingencyRangeOfPossibleLossMinimum
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_StandbyLettersOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_SuretyBondMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LossContingenciesByNatureOfContingencyAxis=acm_DoeDeactivationDemolitionAndRemovalProjectMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=acm_DepartmentOfEnergyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=acm_WgiOhioAndDoeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=acm_WashingtonGroupInternationalMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LossContingenciesByNatureOfContingencyAxis=acm_WorldBankReviewAsiaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Commitments and Contingencies (Details) $ in Millions	1 Months Ended	6 Months Ended
Commitments and Contingencies (Details) $ in Millions	Dec. 31, 2014 USD ($)	Mar. 31, 2017 USD ($) item	Sep. 30, 2016 USD ($)
Standby letter of credit
Commitments and Contingencies
Contingency liability of general and professional liability insurance programs and for payment and performance guarantees		$ 462.2	$ 474.5
Surety Bond
Commitments and Contingencies
Surety bonds issued		3,600.0	$ 3,300.0
DOE Deactivation, Demolition, And Removal Project \| Department of Energy
Commitments and Contingencies
Projects costs, maximum		106.0
DOE Deactivation, Demolition, And Removal Project \| WGI Ohio and DOE
Commitments and Contingencies
Projects costs, minimum		106.0
Projects costs, maximum		146.0
DOE Deactivation, Demolition, And Removal Project \| WGI Ohio
Commitments and Contingencies
Projects costs, minimum		$ 146.0
Claim to DEO, including additional fees	$ 103.0
Possible costs which may exceed contracted amounts	$ 100.0
World Bank Review Asia
Commitments and Contingencies
Affiliates for which inspections conducted of accounts and records (in number) \| item		2