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Share-Based Payments - Incentive Plans (Details) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
 12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2016
Stock Incentive Plans      
Compensation expense related to employer contributions to purchase AECOM stock under defined contribution plans, including the DCP $ 32.3 $ 32.9 $ 26.8
Employee Stock Option      
Stock Incentive Plans      
Securities available for future issuance (in shares) 12.6    
Expiration term of unexercised options 7 years    
Number of Options      
Balance at the beginning of the period (in shares) 0.7 0.9 1.3
Exercised (in shares) (0.1) (0.2) (0.4)
Balance at the end of the period (in shares) 0.6 0.7 0.9
Exercisable at the end of the period (in shares)   0.1 0.3
Stock options, Weighted Average Exercise Price      
Balance at the beginning of the period (in dollars per share) $ 31.11 $ 30.36 $ 28.26
Exercised (in dollars per share) 27.79 26.42 23.96
Balance at the end of the period (in dollars per share) $ 31.62 31.11 30.36
Exercisable at the end of the period (in dollars per share)   $ 27.79 $ 26.99