Revenue Recognition - Disaggregated revenues (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Revenue Recognition
Total revenue	$ 3,266.7	$ 3,313.2
Cost reimbursable
Revenue Recognition
Total revenue	1,307.6	1,327.4
Guaranteed maximum price
Revenue Recognition
Total revenue	1,067.7	1,197.9
Fixed price
Revenue Recognition
Total revenue	891.4	787.9
Americas.
Revenue Recognition
Total revenue	2,464.2	2,557.5
EMEA
Revenue Recognition
Total revenue	440.9	414.7
Asia Pacific
Revenue Recognition
Total revenue	$ 361.6	$ 341.0