Segment Information (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2025 USD ($)	Sep. 30, 2024 USD ($)	Sep. 30, 2025 USD ($) segment	Sep. 30, 2024 USD ($)	Dec. 31, 2024 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | segment			3
Segments:
Total revenue	$ 2,494,000,000	$ 2,628,800,000	$ 7,353,400,000	$ 7,834,700,000
Revenue before billable expenses	2,135,600,000	2,242,700,000	6,304,600,000	6,752,700,000
Severance expense	44,300,000	0	177,700,000	0
Office and other direct expenses	304,500,000	327,100,000	948,900,000	1,007,600,000
Restructuring charges	129,500,000	500,000	450,800,000	1,400,000
Segment EBITA	350,700,000	407,400,000	784,700,000	1,020,000,000
Amortization of acquired intangibles	23,800,000	20,300,000	65,300,000	61,400,000
Goodwill, Impairment Loss	0.0	232,100,000	0	232,100,000
Lease impairment costs	14,600,000	400,000	108,000,000.0	1,300,000
Total assets	16,965,300,000		16,965,300,000		$ 18,325,800,000
OPERATING INCOME	219,000,000.0	132,900,000	420,700,000	635,300,000
Total (expenses) and other income	(30,300,000)	(23,400,000)	(108,400,000)	(69,500,000)
INCOME BEFORE INCOME TAXES	188,700,000	109,500,000	312,300,000	565,800,000
MD&E
Segments:
Total revenue	954,100,000	1,041,500,000	2,812,100,000	3,083,200,000
Revenue before billable expenses	938,800,000	1,025,200,000	2,773,900,000	3,043,800,000
Base salaries, benefits and tax	498,400,000	548,500,000	1,483,100,000	1,697,900,000
Incentive expense	21,600,000	36,200,000	102,000,000.0	104,300,000
Severance expense	500,000	7,000,000.0	2,300,000	43,400,000
Temporary help	19,000,000.0	22,300,000	54,700,000	71,300,000
Office and other direct expenses	164,000,000.0	183,100,000	509,400,000	564,500,000
Depreciation and amortization	22,600,000	27,600,000	70,100,000	81,000,000.0
Restructuring charges	37,100,000	500,000	113,300,000	800,000
Other segment items	17,400,000	20,800,000	41,100,000	54,700,000
Segment EBITA	173,500,000	195,500,000	436,100,000	465,300,000
Amortization of acquired intangibles	22,800,000	19,200,000	62,100,000	57,600,000
Goodwill, Impairment Loss		232,100,000		232,100,000
Capital expenditures	14,400,000	18,200,000	42,000,000.0	57,200,000
Lease impairment costs	12,500,000	400,000	32,200,000	600,000
Total assets	9,652,700,000		9,652,700,000		10,253,000,000
IA&C
Segments:
Total revenue	940,100,000	942,900,000	2,753,000,000	2,913,100,000
Revenue before billable expenses	848,100,000	849,400,000	2,490,600,000	2,647,100,000
Base salaries, benefits and tax	523,000,000.0	535,900,000	1,586,400,000	1,665,300,000
Incentive expense	17,000,000.0	7,100,000	67,000,000.0	57,100,000
Severance expense	2,500,000	14,500,000	6,500,000	51,800,000
Temporary help	39,200,000	33,700,000	110,700,000	105,600,000
Office and other direct expenses	99,800,000	101,400,000	312,800,000	315,500,000
Depreciation and amortization	11,400,000	12,700,000	35,200,000	37,800,000
Restructuring charges	24,500,000	0	139,000,000.0	300,000
Other segment items	104,900,000	104,100,000	296,800,000	300,800,000
Segment EBITA	117,800,000	133,500,000	198,600,000	378,900,000
Amortization of acquired intangibles	600,000	800,000	2,200,000	2,600,000
Goodwill, Impairment Loss		0		0
Capital expenditures	6,500,000	7,600,000	15,900,000	16,500,000
Lease impairment costs	700,000		40,000,000.0	300,000
Total assets	4,508,100,000		4,508,100,000		4,545,000,000
SC&E
Segments:
Total revenue	599,800,000	644,400,000	1,788,300,000	1,838,400,000
Revenue before billable expenses	348,700,000	368,100,000	1,040,100,000	1,061,800,000
Base salaries, benefits and tax	210,600,000	216,800,000	638,100,000	656,700,000
Incentive expense	7,300,000	6,600,000	26,400,000	28,700,000
Severance expense	1,300,000	3,400,000	2,400,000	12,200,000
Temporary help	11,800,000	11,800,000	34,000,000.0	33,700,000
Office and other direct expenses	40,700,000	42,600,000	126,700,000	127,600,000
Depreciation and amortization	2,900,000	3,400,000	9,400,000	10,500,000
Restructuring charges	11,600,000	0	41,100,000	300,000
Other segment items	254,200,000	281,400,000	760,200,000	792,900,000
Segment EBITA	59,400,000	78,400,000	150,000,000.0	175,800,000
Amortization of acquired intangibles	400,000	300,000	1,000,000.0	1,200,000
Goodwill, Impairment Loss		0		0
Capital expenditures	2,400,000	5,400,000	3,600,000	8,200,000
Lease impairment costs	1,300,000		12,200,000	300,000
Total assets	1,701,200,000		1,701,200,000		1,758,900,000
Corporate and Other
Segments:
Restructuring charges	56,300,000		157,400,000
Segment EBITA	(107,900,000)	$ (22,100,000)	(298,700,000)	$ (91,200,000)
Lease impairment costs	100,000		23,400,000
Total assets	$ 1,103,300,000		$ 1,103,300,000		$ 1,768,900,000