Document and Entity Information - shares
3 Months Ended
Dec. 31, 2018
Jan. 31, 2019
Document and Entity Information    
Entity Registrant Name AECOM  
Entity Central Index Key 0000868857  
Document Type 10-Q  
Document Period End Date Dec. 31, 2018  
Amendment Flag false  
Current Fiscal Year End Date --09-30  
Entity Current Reporting Status Yes  
Entity Filer Category Large Accelerated Filer  
Entity Common Stock, Shares Outstanding   156,094,835
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q1