Revenue Recognition - Disaggregated revenues (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Mar. 31, 2022	Mar. 31, 2021
Revenue Recognition
Total revenue	$ 3,213.7	$ 3,265.5	$ 6,480.4	$ 6,578.7
Cost reimbursable
Revenue Recognition
Total revenue	1,378.9	1,405.2	2,686.5	2,732.6
Guaranteed maximum price
Revenue Recognition
Total revenue	1,004.9	1,015.2	2,072.6	2,213.1
Fixed price
Revenue Recognition
Total revenue	829.9	845.1	1,721.3	1,633.0
Americas.
Revenue Recognition
Total revenue	2,400.5	2,469.1	4,864.6	5,026.6
EMEA
Revenue Recognition
Total revenue	457.6	435.4	898.5	850.2
Asia Pacific
Revenue Recognition
Total revenue	$ 355.6	$ 361.0	$ 717.3	$ 701.9