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Significant Accounting Policies (Details) (USD $) In Billions, unless otherwise specified	12 Months Ended
	Sep. 30, 2014 item	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2011
Fiscal Year
Fiscal year period		364 days	364 days	364 days
Revenue Recognition
Other direct costs	$ 3.5	$ 3.2	$ 3.0
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%

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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

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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

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- Details

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- 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

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- Details

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- Definition

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

+ References

No definition available.

+ Details

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Namespace Prefix:	acm_
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- Definition

Represents the number of types of fixed price contracts.

+ References

No definition available.

+ Details

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- Details

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- 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

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