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Commitments and Contingencies (Details) AUD in Millions, $ in Millions			1 Months Ended	9 Months Ended	12 Months Ended
	May 31, 2016 USD ($)	Jul. 10, 2015 USD ($)	Dec. 31, 2014 USD ($)	Jun. 30, 2016 USD ($) item	Sep. 30, 2006 AUD	Sep. 30, 2015 USD ($)
Standby Letters of Credit
Commitments and Contingencies
Contingency liability of general and professional liability insurance programs and for payment and performance guarantees				$ 487.5		$ 436.5
Surety Bond
Commitments and Contingencies
Surety bonds issued				2,900.0		$ 2,300.0
DOE Deactivation Demolition And Removal Project | URS Corporation | Washington Group International
Commitments and Contingencies
Projects costs, maximum				146.0
DOE Deactivation Demolition And Removal Project | Department of Energy
Commitments and Contingencies
Projects costs, minimum				106.0
DOE Deactivation Demolition And Removal Project | Department of Energy | URS Corporation | Washington Group International
Commitments and Contingencies
Projects costs, minimum				106.0
Projects costs, maximum				$ 146.0
Amount recovery (including additional Fees)			$ 103.0
Minimum cost to complete the project and promised amenities			$ 100.0
AECOM Australia
Commitments and Contingencies
Proceeds from initial public offering (IPO) | AUD					AUD 700
Bank loan | AUD					AUD 1,400
Number of initial public offering investors | item				770
Payments for legal settlements		$ 205.0
Damages claimed by applicants	$ 91.0

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