Segment Data - Schedule of summarized financial information by segment (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Segment Reporting Information [Line Items]
Revenue	$ 2,284.6	$ 2,184.8
Cost of gross contract reimbursables	743.9	687.2
Direct employment costs	904.1	863.5
Other direct costs	252.3	281.8
Indirect and overhead employment costs	173.8	166.5
Other indirect and overhead costs	132.0	129.5
Other segment items	(17.7)	(21.8)
Adjusted EBITDA	96.2	78.1
Americas
Segment Reporting Information [Line Items]
Revenue	1,688.3	1,621.0
Cost of gross contract reimbursables	589.5	575.6
Direct employment costs	712.4	671.3
Other direct costs	108.3	131.0
Indirect and overhead employment costs	116.4	108.5
Other indirect and overhead costs	93.2	89.1
Other segment items	(10.8)	(18.9)
Adjusted EBITDA	79.3	64.4
EMEA
Segment Reporting Information [Line Items]
Revenue	205.0	222.4
Cost of gross contract reimbursables	32.4	28.5
Direct employment costs	92.7	94.9
Other direct costs	24.9	33.9
Indirect and overhead employment costs	29.1	30.6
Other indirect and overhead costs	25.0	25.4
Other segment items	(1.1)	0.1
Adjusted EBITDA	2.0	9.0
APAC
Segment Reporting Information [Line Items]
Revenue	391.3	341.4
Cost of gross contract reimbursables	122.0	83.1
Direct employment costs	99.0	97.3
Other direct costs	119.1	116.9
Indirect and overhead employment costs	28.3	27.4
Other indirect and overhead costs	13.8	15.0
Other segment items	(5.8)	(3.0)
Adjusted EBITDA	$ 14.9	$ 4.7