Revenue - Major Geographical Area (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Total revenues:
Total revenue	$ 2,536.8	$ 2,710.0	$ 4,859.4	$ 5,205.9
Revenue before billable expenses	2,172.7	2,327.1	4,169.0	4,510.0
MD&E
Total revenues:
Total revenue	957.4	1,069.8	1,858.3	2,043.2
Revenue before billable expenses	947.7	1,058.7	1,835.4	2,020.1
IA&C
Total revenues:
Total revenue	949.3	1,005.0	1,812.6	1,968.7
Revenue before billable expenses	860.5	914.9	1,642.2	1,796.2
SC&E
Total revenues:
Total revenue	630.1	635.2	1,188.5	1,194.0
Revenue before billable expenses	364.5	353.5	691.4	693.7
United States
Total revenues:
Total revenue	1,645.1	1,755.8	3,219.0	3,417.8
Revenue before billable expenses	1,426.3	1,525.5	2,784.5	3,001.8
United States | MD&E
Total revenues:
Total revenue	613.1	680.5	1,219.7	1,318.2
Revenue before billable expenses	611.9	677.8	1,213.6	1,311.3
United States | IA&C
Total revenues:
Total revenue	578.7	627.0	1,121.6	1,251.6
Revenue before billable expenses	553.2	599.3	1,069.6	1,200.0
United States | SC&E
Total revenues:
Total revenue	453.3	448.3	877.7	848.0
Revenue before billable expenses	261.2	248.4	501.3	490.5
United Kingdom
Total revenues:
Total revenue	213.2	237.5	395.5	448.4
Revenue before billable expenses	178.0	193.7	337.4	371.7
Continental Europe
Total revenues:
Total revenue	242.3	229.5	437.5	433.4
Revenue before billable expenses	202.5	199.2	371.8	378.7
Asia Pacific
Total revenues:
Total revenue	173.0	202.6	320.1	375.7
Revenue before billable expenses	140.2	167.3	259.5	310.1
Latin America
Total revenues:
Total revenue	93.5	110.2	172.6	202.8
Revenue before billable expenses	86.8	102.2	159.3	189.3
Other
Total revenues:
Total revenue	169.7	174.4	314.7	327.8
Revenue before billable expenses	138.9	139.2	256.5	258.4
Total International
Total revenues:
Total revenue	891.7	954.2	1,640.4	1,788.1
Revenue before billable expenses	746.4	801.6	1,384.5	1,508.2
Total International | MD&E
Total revenues:
Total revenue	344.3	389.3	638.6	725.0
Revenue before billable expenses	335.8	380.9	621.8	708.8
Total International | IA&C
Total revenues:
Total revenue	370.6	378.0	691.0	717.1
Revenue before billable expenses	307.3	315.6	572.6	596.2
Total International | SC&E
Total revenues:
Total revenue	176.8	186.9	310.8	346.0
Revenue before billable expenses	$ 103.3	$ 105.1	$ 190.1	$ 203.2