No definition available.

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

- 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

- 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

- References

No definition available.

+ Details

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

- Definition

Represents the entity's fiscal year period 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_FiscalYearPeriod
Namespace Prefix:	acm_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

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

- Definition

Represents the length of fiscal year of the company 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_LengthOfFiscalYear
Namespace Prefix:	acm_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the number of cost reimbursable contracts that the entity enters into.

+ References

No definition available.

+ Details

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

- Definition

Represents the number of major types of cost-plus contracts that the entity enters into.

+ References

No definition available.

+ Details

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

- Definition

Represents the number of types of fixed price contracts.

+ References

No definition available.

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Other costs incurred and are directly related to services rendered by an entity during the reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.2(d))

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

+ Details

Name:	us-gaap_OtherCostOfServices
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- References

No definition available.

+ Details

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

- Definition

Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

+ References

No definition available.

+ Details

Name:	us-gaap_PropertyPlantAndEquipmentUsefulLife
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

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

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Significant Accounting Policies (Details) $ in Billions	12 Months Ended
Significant Accounting Policies (Details) $ in Billions	Sep. 30, 2015 USD ($) item	Sep. 30, 2014 USD ($)	Sep. 30, 2013 USD ($)
Fiscal Year
Length of Fiscal Year	364 days	371 days	364 days
Revenue Recognition
Other direct costs \| $	$ 8.3	$ 3.5	$ 3.2
Number of cost reimbursable contracts	2
Cost-Plus Contracts
Number of major types of cost-plus contracts the Company enters into	2
Fixed Price Contracts
Number of types of fixed price contracts	2
Pension Plans
Amortization period of the difference between actual return and expected return	5 years
Specified percentage to be exceeded for amortization of cumulative, net unrecognized gains or losses (as a percent)	10.00%
Minimum
Fiscal Year
Fiscal year period	364 days
Property and Equipment
Estimated useful lives for equipment furniture and fixtures	3 years
Maximum
Fiscal Year
Fiscal year period	371 days
Property and Equipment
Estimated useful lives for equipment furniture and fixtures	10 years