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Major Clients (Details) (Sales Revenue, Services, Net [Member], Customer Concentration Risk [Member])	12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2012
Percentage of total revenue
Period for which criteria of threshold percentage for disclosure not met in past	5 years
Maximum
Percentage of total revenue
Percentage of revenue accounted for by no other single client (as a percent)	10.00%
Agencies of U.S. Federal Government [Member]
Percentage of total revenue
Percentage of the Company's revenue (as a percent)	15.00%	18.00%	18.00%
Agencies of U.S. Federal Government [Member] | Management Support Services [Member]
Percentage of total revenue
Percentage of the Company's revenue (as a percent)	3.00%	4.00%	4.00%

X

- Definition

Represents the period for which criteria of threshold percentage for disclosure not met 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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Period Type:	duration

X

- Definition

Threshold percentage which the entity uses for disclosure.

+ References

No definition available.

+ Details

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Namespace Prefix:	acm_
Data Type:	num:percentItemType
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X

- Details

Name:	us-gaap_ConcentrationRiskLineItems
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Data Type:	xbrli:stringItemType
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Period Type:	duration

X

- Definition

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 21

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=28364263&loc=d3e13537-108611



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 275

-SubTopic 10

-Section 50

-Paragraph 20

-URI http://asc.fasb.org/extlink&oid=6927468&loc=d3e6404-108592



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 20

-URI http://asc.fasb.org/extlink&oid=28364263&loc=d3e13531-108611



Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 275

-SubTopic 10

-Section 50

-Paragraph 18

-URI http://asc.fasb.org/extlink&oid=6927468&loc=d3e6351-108592

+ Details

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