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Revenue Recognition - Disaggregated revenues (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Revenue Recognition
Total revenue	$ 3,408.4	$ 3,189.7	$ 9,987.1	$ 9,671.0
Cost reimbursable
Revenue Recognition
Total revenue	1,309.2	1,369.3	4,041.8	4,252.5
Guaranteed maximum price
Revenue Recognition
Total revenue	1,229.9	829.8	3,443.0	2,706.4
Fixed price
Revenue Recognition
Total revenue	869.3	990.6	2,502.3	2,712.1
Americas
Revenue Recognition
Total revenue	2,619.0	2,471.8	7,645.7	7,400.5
Europe, Middle East, Africa
Revenue Recognition
Total revenue	401.3	359.9	1,201.8	1,194.9
Asia Pacific
Revenue Recognition
Total revenue	$ 388.1	$ 358.0	$ 1,139.6	$ 1,075.6

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- Definition

Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

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- Definition

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