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Share-based Payments (Details) (USD $)
In Millions, except Per Share data, unless otherwise specified
3 Months Ended 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Sep. 30, 2014
Compensation expense      
Granted (in shares) 2.6us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod    
Granted (in dollars per share) $ 30.04us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue    
Recognized compensation expense $ 62.3us-gaap_AllocatedShareBasedCompensationExpense $ 10.9us-gaap_AllocatedShareBasedCompensationExpense  
Accelerated compensation expense 43.9us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAcceleratedCompensationCost    
Accelerated settlement Of Common Stock 17.6acm_ShareBasedCompensationArrangementByShareBasedPaymentAwardSettlementCommonStock    
Award settlement Of Cash 26.3acm_ShareBasedCompensationArrangementByShareBasedPaymentAwardSettlementOfCash    
Unrecognized compensation expense $ 175.2us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized   $ 62.4us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
Vesting period 3 years   3 years
Employee Stock Option [Member]      
Number of Options      
Balance at the beginning of the period (in shares) 1.6us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
1.6us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
1.6us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
Granted (in shares) 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
   
Exercised (in shares) (0.1)us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
(0.1)us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
 
Cancelled (in shares) 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
(0.1)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
 
Balance at the end of the period (in shares) 1.5us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
1.4us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
 
Vested and expected to vest in the future at the end of the period (in shares) 1.5us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
1.4us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
 
Stock options, Weighted Average Exercise Price      
Balance at the beginning of the period (in dollars per share) $ 27.69us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
$ 24.73us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
$ 24.73us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
Granted (in dollars per share) $ 0.00us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
   
Exercised (in dollars per share) $ 26.24us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
$ 17.23us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
 
Cancelled (in dollars per share) $ 27.55us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriodWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
$ 26.74us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriodWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
 
Balance at the end of the period (in dollars per share) $ 27.75us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
$ 25.15us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
 
Vested and expected to vest in the future at the end of the period (in dollars per share) $ 27.75us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
$ 25.15us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
 
Performance Earnings Program [Member]      
Compensation expense      
Vesting period 3 years    
Weighted average grant-date fair value of stock options granted (in dollars per share) $ 32.38us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= acm_PerformanceEarningsProgramMember
$ 29.22us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= acm_PerformanceEarningsProgramMember
 
Restricted Stock Units (RSUs) [Member]      
Compensation expense      
Weighted average grant-date fair value of stock options granted (in dollars per share) $ 31.06us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
$ 29.19us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember