The percentage of gross profit to total net revenue from services rendered.

No definition available.

Name:	acm_GrossProfitAsPercentageOfSalesRevenueServicesNet
Namespace Prefix:	acm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of write-down of assets recognized in the income statement. Includes, but is not limited to, losses from tangible assets, intangible assets and goodwill.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 45
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=68055307&loc=d3e2420-110228

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

+ Details

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

- Definition

Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(12))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(11))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

Name:	us-gaap_Assets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This element represents acquisition-related costs incurred to effect a business combination which costs have been expensed during the period. Such costs include finder's fees; advisory, legal, accounting, valuation, and other professional or consulting fees; general administrative costs, including the costs of maintaining an internal acquisitions department; and may include costs of registering and issuing debt and equity securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 25
-Paragraph 23
-URI http://asc.fasb.org/extlink&oid=84229725&loc=d3e1043-128460

+ Details

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

- Definition

Total costs related to services rendered by an entity during the reporting period.

+ References

+ Details

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

- Definition

Amount before tax of gain (loss) recognized on the sale or disposal of a disposal group. Excludes discontinued operations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=68055307&loc=d3e2443-110228

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=84176398&loc=d3e2941-110230

+ Details

Name:	us-gaap_DisposalGroupNotDiscontinuedOperationGainLossOnDisposal
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The aggregate total of expenses of managing and administering the affairs of an entity, including affiliates of the reporting entity, which are not directly or indirectly associated with the manufacture, sale or creation of a product or product line.

+ References

+ Details

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

- Definition

Aggregate revenue less cost of goods and services sold or operating expenses directly attributable to the revenue generation activity.

+ References

+ Details

Name:	us-gaap_GrossProfit
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

This item represents the entity's proportionate share for the period of the net income (loss) of its investee (such as unconsolidated subsidiaries and joint ventures) to which the equity method of accounting is applied. This item includes income or expense related to stock-based compensation based on the investor's grant of stock to employees of an equity method investee.

+ 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(12))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(10))
-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66022249&loc=d3e33749-111570

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(13)(f))
-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

+ Details

Name:	us-gaap_IncomeLossFromEquityMethodInvestments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

+ References

No definition available.

+ Details

Name:	us-gaap_NumberOfReportableSegments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

The net result for the period of deducting operating expenses from operating revenues.

+ References

No definition available.

+ Details

Name:	us-gaap_OperatingIncomeLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Aggregate revenue during the period from services rendered in the normal course of business, after deducting allowances and discounts.

+ References

+ Details

Name:	us-gaap_SalesRevenueServicesNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_ConsolidationItemsAxis=us-gaap_CorporateNonSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=acm_DesignConsultingServicesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=acm_ConstructionServicesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=acm_ManagementSupportServicesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementScenarioAxis=us-gaap_RestatementAdjustmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=acm_AecomCapitalMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Reportable Segments (Details) $ in Thousands	3 Months Ended								12 Months Ended
Reportable Segments (Details) $ in Thousands	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2018 USD ($) segment	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)
Summarized financial information concerning the Company's reportable segments
Number of reportable segments \| segment									4
Cost of revenue.	$ 5,117,800	$ 4,962,800	$ 4,649,700	$ 4,774,600	$ 4,686,300	$ 4,386,300	$ 4,258,800	$ 4,188,300	$ 19,504,863	$ 17,519,682	$ 16,768,001
Revenue	5,305,800	5,148,000	4,790,900	4,910,800	4,856,400	4,561,500	4,427,200	4,358,300	20,155,512	18,203,402	17,410,825
Gross profit	188,000	185,200	141,200	136,200	170,100	175,200	168,400	170,000	650,649	683,720	642,824
Equity in earnings of joint ventures	25,500	12,800	13,100	29,700	31,900	66,500	21,800	21,400	81,133	141,582	104,032
General and administrative expenses	(35,700)	(35,100)	(30,200)	(34,700)	(36,900)	(34,000)	(29,900)	(32,600)	(135,787)	(133,309)	(115,088)
Impairment on assets held for sale, including goodwill			(168,200)						(168,178)
Acquisition and integration expenses					(3,300)		(20,000)	(15,400)		(38,709)	(213,642)
(Loss) gain on disposal activities	(800)	(2,100)					600		(2,949)	572	(42,589)
Operating income (loss)	177,000	$ 160,800	$ (44,100)	$ 131,200	161,800	$ 207,700	$ 140,900	$ 143,400	424,868	653,856	375,537
Total assets	14,681,131				14,396,956				$ 14,681,131	$ 14,396,956	$ 13,669,900
Gross profit as a % of revenue									3.20%	3.80%	3.70%
Corporate
Summarized financial information concerning the Company's reportable segments
General and administrative expenses									$ (124,500)	$ (124,700)	$ (109,100)
Acquisition and integration expenses										(38,700)	(213,600)
Operating income (loss)									(124,500)	(163,400)	(322,700)
Total assets	613,500				386,200				613,500	386,200	586,200
Design and Consulting Services
Summarized financial information concerning the Company's reportable segments
Revenue									8,223,100	7,566,800	7,655,800
Gross profit									439,200	394,800	382,500
Equity in earnings of joint ventures									15,800	16,400	8,900
(Loss) gain on disposal activities										600
Operating income (loss)									455,000	411,800	391,400
Total assets	7,013,800				6,992,600				$ 7,013,800	$ 6,992,600	$ 6,655,700
Gross profit as a % of revenue									5.30%	5.20%	5.00%
Construction Services
Summarized financial information concerning the Company's reportable segments
Revenue									$ 8,238,900	$ 7,295,600	$ 6,371,100
Gross profit									40,400	92,900	25,400
Equity in earnings of joint ventures									21,500	22,400	18,200
Impairment on assets held for sale, including goodwill									(168,200)
(Loss) gain on disposal activities									(2,900)		(42,600)
Operating income (loss)									(109,200)	115,300	1,000
Total assets	4,212,000				4,114,500				$ 4,212,000	$ 4,114,500	$ 3,556,200
Gross profit as a % of revenue									0.50%	1.30%	0.40%
Management Services
Summarized financial information concerning the Company's reportable segments
Revenue									$ 3,693,500	$ 3,341,000	$ 3,383,900
Gross profit									171,000	196,000	234,900
Equity in earnings of joint ventures									28,600	45,100	76,900
Operating income (loss)									199,600	241,100	311,800
Total assets	2,701,200				2,704,600				$ 2,701,200	$ 2,704,600	$ 2,692,700
Gross profit as a % of revenue									4.60%	5.90%	6.90%
Management Services \| Restatement Adjustment
Summarized financial information concerning the Company's reportable segments
Cost of revenue.										$ 124,000
Revenue										130,000
AECOM Capital
Summarized financial information concerning the Company's reportable segments
Equity in earnings of joint ventures									$ 15,200	57,700
General and administrative expenses									(11,200)	(8,700)	$ (6,000)
Operating income (loss)									4,000	49,000	(6,000)
Total assets	$ 140,600				$ 199,100				$ 140,600	$ 199,100	$ 179,100