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Business Acquisitions, Goodwill and Intangible Assets - URS Acquisition (Details) $ in Millions		12 Months Ended
	Oct. 17, 2014 USD ($)	Sep. 30, 2018 USD ($) item	Sep. 30, 2017 USD ($) item	Sep. 30, 2016 USD ($)
Business acquisition
Number of acquisitions | item		1	2
Total consideration		$ 5.6	$ 164.4
Amortization of Intangible Assets		96.7	102.7	$ 202.4
URS
Business acquisition
Estimated Legal settlements and uninsured legal damages	$ 240.0
Amortization of Intangible Assets		7.1	9.4
Margin fair value liability		20.6	8.6
Income from operations related to margin fair value liability		21.1	6.3
URS | Customer relationships, contracts and backlog
Business acquisition
Acquired backlog and customer relationship assets	$ 973.8
URS | Customer relationships, contracts and backlog | Minimum
Business acquisition
Useful life (in years)	1 year
URS | Customer relationships, contracts and backlog | Maximum
Business acquisition
Useful life (in years)	11 years
URS | Customer relationships, contracts and backlog | Weighted Average
Business acquisition
Useful life (in years)	3 years
URS | Cost of revenue
Business acquisition
Amortization of Intangible Assets		68.4	83.6
URS | Non-Controlling Interests
Business acquisition
Amortization of Intangible Assets		$ (8.5)	$ (8.5)

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