Segment Information (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025 USD ($)	Jun. 30, 2024 USD ($)	Jun. 30, 2025 USD ($) segment	Jun. 30, 2024 USD ($)	Dec. 31, 2024 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | segment			3
Segments:
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
Severance expense	53.3	0.0	133.4	0.0
Office and other direct expenses	325.2	358.4	644.4	680.5
Restructuring charges	118.0	0.3	321.3	0.9
Segment EBITA	341.4	367.6	434.0	612.6
Amortization of acquired intangibles	21.1	20.4	41.5	41.1
Lease impairment costs	23.1	0.3	93.4	0.9
Total assets	17,027.1		17,027.1		$ 18,325.8
OPERATING INCOME	243.7	318.2	201.7	502.4
Total (expenses) and other income	(25.7)	(22.5)	(78.1)	(46.1)
INCOME BEFORE INCOME TAXES	218.0	295.7	123.6	456.3
MD&E
Segments:
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
Base salaries, benefits and tax	487.5	570.5	986.0	1,150.5
Incentive expense	38.4	39.6	81.1	68.7
Severance expense	1.1	14.2	1.8	36.5
Temporary help	18.3	24.0	35.7	49.0
Office and other direct expenses	172.5	202.4	346.1	382.7
Depreciation and amortization	23.8	27.0	47.5	53.5
Restructuring charges	22.4	0.3	76.3	0.3
Other segment items	8.6	16.4	23.6	33.9
Segment EBITA	184.8	175.4	260.2	268.1
Amortization of acquired intangibles	20.1	19.1	39.3	38.4
Capital expenditures	14.6	17.5	27.6	39.0
Lease impairment costs	0.7	0.3	19.7	0.3
Total assets	9,508.0		9,508.0		10,253.7
IA&C
Segments:
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
Base salaries, benefits and tax	526.4	556.3	1,062.1	1,128.3
Incentive expense	18.5	30.5	49.3	49.4
Severance expense	2.1	17.0	4.0	37.2
Temporary help	38.1	35.5	71.5	71.9
Office and other direct expenses	108.7	111.8	212.3	212.8
Depreciation and amortization	11.7	12.6	23.8	25.0
Restructuring charges	59.2	0.0	114.5	0.3
Other segment items	100.2	102.6	192.0	196.7
Segment EBITA	84.4	138.7	83.1	247.1
Amortization of acquired intangibles	0.7	0.9	1.6	1.8
Capital expenditures	6.0	6.5	9.4	8.9
Lease impairment costs	20.7		39.3	0.3
Total assets	4,618.2		4,618.2		4,544.3
SC&E
Segments:
Total revenue	630.1	635.2	1,188.5	1,194.0
Revenue before billable expenses	364.5	353.5	691.4	693.7
Base salaries, benefits and tax	212.9	218.8	427.5	439.9
Incentive expense	7.8	12.5	19.1	22.1
Severance expense	0.3	4.1	1.1	8.8
Temporary help	11.9	10.8	22.2	21.9
Office and other direct expenses	44.0	44.2	86.0	85.0
Depreciation and amortization	3.1	3.6	6.5	7.1
Restructuring charges	7.7	0.0	29.4	0.3
Other segment items	270.2	287.7	506.0	511.5
Segment EBITA	72.2	53.5	90.7	97.4
Amortization of acquired intangibles	0.3	0.4	0.6	0.9
Capital expenditures	0.7	2.0	1.2	2.8
Lease impairment costs	1.5		10.9	0.3
Total assets	1,729.0		1,729.0		1,758.9
Corporate and Other
Segments:
Restructuring charges	28.7		101.1
Segment EBITA	(76.6)	$ (29.0)	(190.8)	$ (69.1)
Lease impairment costs	0.2		23.3
Total assets	$ 1,171.9		$ 1,171.9		$ 1,768.9