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Accounts Receivable-Net (Details) (USD $)
 3 Months Ended
Dec. 31, 2014
Sep. 30, 2014
Accounts Receivable-Net    
Billed $ 2,375,900,000acm_BilledReceivablesCurrent $ 1,248,400,000acm_BilledReceivablesCurrent
Unbilled 2,148,600,000us-gaap_UnbilledReceivablesCurrent 1,214,800,000us-gaap_UnbilledReceivablesCurrent
Contract retentions 442,000,000us-gaap_ContractReceivableRetainageDueOneYearOrLess 263,900,000us-gaap_ContractReceivableRetainageDueOneYearOrLess
Total accounts receivable-gross 4,966,500,000us-gaap_AccountsReceivableGrossCurrent 2,727,100,000us-gaap_AccountsReceivableGrossCurrent
Allowance for doubtful accounts (92,600,000)us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent (72,100,000)us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent
Total accounts receivable -net 4,873,911,000us-gaap_ReceivablesNetCurrent 2,654,976,000us-gaap_ReceivablesNetCurrent
Period in which unbilled receivables are expected to be billed and collected 12 months  
Sale of trade receivables, outstanding $ 129,300,000us-gaap_ReceivablesHeldForSaleAmount $ 111,900,000us-gaap_ReceivablesHeldForSaleAmount