Segment Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024	Dec. 31, 2024
Segments:
Total revenue	$ 2,322.6	$ 2,495.9
Net revenue	1,996.3	2,182.9
Severance and termination costs	80.1	0.0
Office and other direct expenses	319.2	322.1
Segment EBITA	92.6	245.0
Amortization of acquired intangibles	20.4	20.7
Capital expenditures	21.5	35.1
Total assets	17,125.5		$ 18,325.8
Operating income	(42.0)	184.2
Total (expenses) and other income	(52.4)	(23.6)
Income before income taxes	(94.4)	160.6
Restructuring charges	203.3	0.6
MD&E
Segments:
Total revenue	902.7	973.3
Net revenue	888.8	961.3
Salary and Wage, Excluding Cost of Good and Service Sold	500.0	580.5
Incentive Fee Expense	42.9	29.1
Severance and termination costs	0.7	22.2
Other Labor-related Expenses	17.1	24.2
Office and other direct expenses	173.9	180.2
Segment EBITA	75.6	93.2
Amortization of acquired intangibles	19.2	19.2
Depreciation	23.6	26.5
Other segment expenses	15.6	17.4
Capital expenditures	13.0	21.5
Total assets	9,565.4		10,248.1
Restructuring charges	53.3	0.0
Lease impairment costs	19.1
IA&C
Segments:
Total revenue	861.5	963.8
Net revenue	780.6	881.4
Salary and Wage, Excluding Cost of Good and Service Sold	534.2	571.5
Incentive Fee Expense	30.6	18.9
Severance and termination costs	1.9	20.3
Other Labor-related Expenses	33.7	37.2
Office and other direct expenses	103.3	101.1
Segment EBITA	(1.5)	107.9
Amortization of acquired intangibles	0.9	1.0
Depreciation	12.2	12.4
Other segment expenses	91.2	94.2
Capital expenditures	3.4	2.4
Total assets	4,541.7		4,549.9
Restructuring charges	55.9	0.3
Lease impairment costs	18.6	0.3
SC&E
Segments:
Total revenue	558.4	558.8
Net revenue	326.9	340.2
Salary and Wage, Excluding Cost of Good and Service Sold	214.6	221.1
Incentive Fee Expense	11.3	9.6
Severance and termination costs	0.8	4.7
Other Labor-related Expenses	10.3	11.1
Office and other direct expenses	42.0	40.8
Segment EBITA	18.5	43.9
Amortization of acquired intangibles	0.3	0.5
Depreciation	3.4	3.5
Other segment expenses	235.8	223.8
Capital expenditures	0.5	0.8
Total assets	1,641.4		1,758.9
Restructuring charges	21.7	0.3
Lease impairment costs	9.4	0.3
Corporate and Other
Segments:
Segment EBITA	(114.2)	$ (40.1)
Total assets	1,377.0		$ 1,768.9
Restructuring charges	72.4
Lease impairment costs	$ 23.1