Represents the consolidated interest coverage ratio.

No definition available.

Name:	acm_ConsolidatedInterestCoverageRatio
Namespace Prefix:	acm_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

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

- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

+ Details

Name:	us-gaap_LettersOfCreditOutstandingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(b),22(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Indebtedness divided by net capital.

+ References

No definition available.

+ Details

Name:	us-gaap_RatioOfIndebtednessToNetCapital1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Details

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

- Details

Name:	us-gaap_DebtInstrumentAxis=acm_CreditAgreement2014Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_BaseRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_EurodollarMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

Name:	srt_CurrencyAxis=currency_USD
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

Debt - 2014 Credit Agreement (Details) $ in Millions, $ in Millions, $ in Millions			12 Months Ended
	Jul. 01, 2018	Mar. 13, 2018 CAD ($)	Sep. 30, 2019 USD ($)	Oct. 01, 2019	Sep. 30, 2018 USD ($)	Mar. 13, 2018 AUD ($)	Mar. 13, 2018 USD ($)	Mar. 31, 2017	Sep. 29, 2016 USD ($)	Oct. 17, 2014 USD ($)
Term loan A
Debt agreements
Borrowing capacity									$ 185.0
2014 Credit Agreement \| Minimum \| Base Rate
Debt agreements
Interest rate, basis spread (as a percent)	0.25%
2014 Credit Agreement \| Minimum \| Euro currency rate
Debt agreements
Interest rate, basis spread (as a percent)	1.25%
2014 Credit Agreement \| Maximum \| Base Rate
Debt agreements
Interest rate, basis spread (as a percent)	1.00%
2014 Credit Agreement \| Maximum \| Euro currency rate
Debt agreements
Interest rate, basis spread (as a percent)	2.00%
2014 Credit Agreement \| Term loan A
Debt agreements
Borrowing capacity		$ 500.0				$ 250.0	$ 510.0		185.0
2014 Credit Agreement \| Term loan A \| Base Rate \| USD
Debt agreements
Interest rate, basis spread (as a percent)		0.50%
2014 Credit Agreement \| Term loan A \| Euro currency rate \| USD
Debt agreements
Interest rate, basis spread (as a percent)		1.50%
2014 Credit Agreement \| Term loan B
Debt agreements
Borrowing capacity							600.0
2014 Credit Agreement \| Term loan B \| Base Rate
Debt agreements
Interest rate, basis spread (as a percent)		0.75%
2014 Credit Agreement \| Term loan B \| Euro currency rate
Debt agreements
Interest rate, basis spread (as a percent)		1.75%
2014 Credit Agreement \| Revolving credit facility
Debt agreements
Borrowing capacity							$ 1,350.0			$ 1,050.0
Outstanding letters of credit			$ 22.8		$ 28.7
Remaining borrowing capacity under credit facility			$ 1,327.2		$ 1,321.3
2014 Credit Agreement \| Revolving credit facility \| Base Rate
Debt agreements
Interest rate, basis spread (as a percent)		0.75%
2014 Credit Agreement \| Revolving credit facility \| Euro currency rate
Debt agreements
Interest rate, basis spread (as a percent)		1.75%
2014 Credit Agreement \| Line of credit
Debt agreements
Borrowing capacity									$ 500.0
Consolidated leverage ratio		4.5	3.4	4.0		4.5	4.5	4.75	5.00
Consolidated interest coverage ratio			4.9