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Accounts Receivable-Net (Details) (USD $)
 6 Months Ended
Mar. 31, 2015
Sep. 30, 2014
Accounts Receivable-Net    
Billed $ 2,365,600,000acm_BilledReceivablesCurrent $ 1,248,400,000acm_BilledReceivablesCurrent
Unbilled 2,093,300,000us-gaap_UnbilledReceivablesCurrent 1,214,800,000us-gaap_UnbilledReceivablesCurrent
Contract retentions 403,000,000us-gaap_ContractReceivableRetainageDueOneYearOrLess 263,900,000us-gaap_ContractReceivableRetainageDueOneYearOrLess
Total accounts receivable-gross 4,861,900,000us-gaap_AccountsReceivableGrossCurrent 2,727,100,000us-gaap_AccountsReceivableGrossCurrent
Allowance for doubtful accounts (76,600,000)us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent (72,100,000)us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent
Total accounts receivable -net 4,785,335,000us-gaap_ReceivablesNetCurrent 2,654,976,000us-gaap_ReceivablesNetCurrent
Period in which unbilled receivables are expected to be billed and collected 12 months  
Total receivables sold that remain outstanding $ 181,200,000acm_ReceivablesSoldThatRemainOutstanding $ 111,900,000acm_ReceivablesSoldThatRemainOutstanding