Segment Operations (Details) $ in Millions	12 Months Ended
	Dec. 31, 2024 USD ($) numberOfPeople	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Schedule of Segment Reporting Information, by Segment [Table]
Number of Reportable Segments | numberOfPeople	3
Total (expenses) and other income	$ (154.1)	$ (74.6)	$ (112.3)
Total revenue	10,691.7	10,889.3	10,927.8
Net revenue	9,187.6	9,400.6	9,449.4
Operating Income (Loss)	1,203.2	1,482.6	1,381.2
Amortization of Intangible Assets	81.8	84.0	84.7
Goodwill, Impairment Loss	232.1	0.0	0.0
Interest expense	(229.9)	(225.6)	(167.9)
Interest income	151.7	140.8	56.6
Other (expense) income, net	(75.9)	10.2	(1.0)
Income before income taxes	1,049.1	1,408.0	1,268.9
Billable expenses	1,504.1	1,488.7	1,478.4
Capital expenditures	141.8	179.3	178.1
Assets	18,325.8	19,267.3
Restructuring Charges	(5.0)	0.1	102.4
Segment EBITA	$ 1,649.9	1,638.9	1,560.4
Segment Reporting Information [Line Items]
Number of Reportable Segments | numberOfPeople	3
Total (expenses) and other income	$ (154.1)	(74.6)	(112.3)
Total revenue	10,691.7	10,889.3	10,927.8
Net revenue	9,187.6	9,400.6	9,449.4
Operating Income (Loss)	1,203.2	1,482.6	1,381.2
Amortization of Intangible Assets	81.8	84.0	84.7
Goodwill, Impairment Loss	232.1	0.0	0.0
Interest expense	(229.9)	(225.6)	(167.9)
Interest income	151.7	140.8	56.6
Other (expense) income, net	(75.9)	10.2	(1.0)
Income before income taxes	1,049.1	1,408.0	1,268.9
Billable expenses	1,504.1	1,488.7	1,478.4
Capital expenditures	141.8	179.3	178.1
Assets	18,325.8	19,267.3
Restructuring Charges	(5.0)	0.1	102.4
Segment EBITA	1,649.9	1,638.9	1,560.4
MD&E
Schedule of Segment Reporting Information, by Segment [Table]
EBITA	847.9	836.9	748.4
Salary and Wage, Excluding Cost of Good and Service Sold	2,207.9	2,358.4	2,286.2
Incentive Fee Expense	128.4	124.4	158.0
Other Labor-related Expenses	85.3	104.4	135.4
Other segment expenses	81.1	92.4	182.8
Amortization	77.2	77.3	72.8
Total revenue	4,274.3	4,417.4	4,401.1
Net revenue	4,213.8	4,348.6	4,315.7
Goodwill, Impairment Loss	232.1
Capital expenditures	75.6	100.2	97.5
Assets	10,248.1	10,737.5
Segment Reporting Information [Line Items]
EBITA	847.9	836.9	748.4
Salary and Wage, Excluding Cost of Good and Service Sold	2,207.9	2,358.4	2,286.2
Incentive Fee Expense	128.4	124.4	158.0
Other Labor-related Expenses	85.3	104.4	135.4
Other segment expenses	81.1	92.4	182.8
Amortization	77.2	77.3	72.8
Total revenue	4,274.3	4,417.4	4,401.1
Net revenue	4,213.8	4,348.6	4,315.7
Goodwill, Impairment Loss	232.1
Capital expenditures	75.6	100.2	97.5
Assets	10,248.1	10,737.5
IA&C
Schedule of Segment Reporting Information, by Segment [Table]
EBITA	542.6	536.8	577.5
Salary and Wage, Excluding Cost of Good and Service Sold	2,187.0	2,244.3	2,225.9
Incentive Fee Expense	95.6	84.9	139.8
Other Labor-related Expenses	142.9	154.0	203.6
Other segment expenses	409.2	414.9	451.5
Amortization	3.1	4.3	7.2
Total revenue	3,907.2	3,979.6	4,121.1
Net revenue	3,544.4	3,611.1	3,747.5
Goodwill, Impairment Loss	0.0
Capital expenditures	29.5	40.7	43.7
Assets	4,549.9	4,790.4
Segment Reporting Information [Line Items]
EBITA	542.6	536.8	577.5
Salary and Wage, Excluding Cost of Good and Service Sold	2,187.0	2,244.3	2,225.9
Incentive Fee Expense	95.6	84.9	139.8
Other Labor-related Expenses	142.9	154.0	203.6
Other segment expenses	409.2	414.9	451.5
Amortization	3.1	4.3	7.2
Total revenue	3,907.2	3,979.6	4,121.1
Net revenue	3,544.4	3,611.1	3,747.5
Goodwill, Impairment Loss	0.0
Capital expenditures	29.5	40.7	43.7
Assets	4,549.9	4,790.4
SC&E
Schedule of Segment Reporting Information, by Segment [Table]
EBITA	259.4	265.2	234.5
Salary and Wage, Excluding Cost of Good and Service Sold	870.9	860.7	836.4
Incentive Fee Expense	33.1	43.3	44.2
Other Labor-related Expenses	45.7	52.4	51.8
Other segment expenses	1,102.1	1,074.6	1,044.3
Amortization	1.5	2.4	4.7
Total revenue	2,510.2	2,492.3	2,405.6
Net revenue	1,429.4	1,440.9	1,386.2
Goodwill, Impairment Loss	0.0
Capital expenditures	12.1	6.4	7.3
Assets	1,758.9	1,800.6
Segment Reporting Information [Line Items]
EBITA	259.4	265.2	234.5
Salary and Wage, Excluding Cost of Good and Service Sold	870.9	860.7	836.4
Incentive Fee Expense	33.1	43.3	44.2
Other Labor-related Expenses	45.7	52.4	51.8
Other segment expenses	1,102.1	1,074.6	1,044.3
Amortization	1.5	2.4	4.7
Total revenue	2,510.2	2,492.3	2,405.6
Net revenue	1,429.4	1,440.9	1,386.2
Goodwill, Impairment Loss	0.0
Capital expenditures	12.1	6.4	7.3
Assets	1,758.9	1,800.6
Corporate Segment and Other Operating Segment
Schedule of Segment Reporting Information, by Segment [Table]
EBITA	(132.8)	(72.3)	(94.5)
Assets	1,768.9	1,938.8
Segment Reporting Information [Line Items]
EBITA	(132.8)	(72.3)	$ (94.5)
Assets	$ 1,768.9	$ 1,938.8