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New Accounting Pronouncements and Changes in Accounting (Details) - Accounting Standards Update April 2015 - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016
Recent Accounting Pronouncements
Unamortized debt issuance costs (noncurrent assets)	$ (54.0)	$ (56.8)
Unamortized debt issuance costs (long term debt)	$ 54.0	$ 56.8

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- Definition

Represents unamortized debt issued costs currently classified as other current assets.

+ References

No definition available.

+ Details

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

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- Definition

Represents information pertaining to unamortized debt issuance costs classified as long-term debt.

+ References

No definition available.

+ Details

Name:	acm_UnamortizedDebtIssuanceCostsLiability
Namespace Prefix:	acm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

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- References

No definition available.

+ Details

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

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- Details

Name:	us-gaap_AdjustmentsForNewAccountingPronouncementsAxis=acm_AccountingStandardsUpdateApril2015Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: