Results by Quarter (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Quarterly Financial Information Disclosure [Abstract]
Net revenue	$ 9,400.6	$ 9,449.4	$ 9,107.9
Billable expenses	1,488.7	1,478.4	1,132.8
Total revenue	10,889.3	10,927.8	10,240.7
Salaries and related expenses	6,243.9	6,258.3	5,975.4
Office and other direct expenses	1,342.5	1,346.4	1,279.6
Billable expense	1,488.7	1,478.4	1,132.8
Cost of services	9,075.1	9,083.1	8,387.8
Depreciation and amortization	264.3	274.0	283.8
Restructuring Charges	0.1	102.4	10.6
Total operating expenses	9,406.7	9,546.6	8,804.5
Operating Income (Loss)	1,482.6	1,381.2	1,436.2
Net losses on sales of businesses	17.9	(11.3)	(19.4)
Tax Adjustments, Settlements, and Unusual Provisions	(64.2)
Total (expenses) and other income	(74.6)	(112.3)	(214.1)
NET INCOME	$ 1,118.1	$ 956.1	$ 972.8
Earnings per share, Basic	$ 2.86	$ 2.40	$ 2.42
Earnings per share, Diluted	$ 2.85	$ 2.37	$ 2.39