Revenue Income Statement (Details) - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
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
OPERATING EXPENSES:
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
Selling, General and Administrative Expense	67.2	87.1	122.3
Depreciation and amortization	264.3	274.0	283.8
Total operating expenses	9,406.7	9,546.6	8,804.5
Operating Income (Loss)	1,482.6	1,381.2	1,436.2
EXPENSES AND OTHER INCOME:
Interest Expense	(225.6)	(167.9)	(170.6)
Interest income	140.8	56.6	27.2
Other income (expense), net	10.2	(1.0)	(70.7)
Total (expenses) and other income	(74.6)	(112.3)	(214.1)
Income before income taxes	1,408.0	1,268.9	1,222.1
Provision for income taxes	291.2	318.4	251.8
Income of consolidated companies	1,116.8	950.5	970.3
Equity in net income of unconsolidated affiliates	1.3	5.6	2.5
NET INCOME	1,118.1	956.1	972.8
Net Income Attributable to Parent	$ 1,098.4	$ 938.0	$ 952.8
Earnings per share, Basic	$ 2.86	$ 2.40	$ 2.42
Earnings per share, Diluted	$ 2.85	$ 2.37	$ 2.39
Weighted-average number of common shares outstanding, Basic	384.1	391.5	393.0
Weighted-average number of common shares outstanding, Diluted	385.9	395.1	398.4
MD&E
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenue	$ 4,326.5	$ 4,296.9	$ 4,117.7
Total revenue	4,394.8	4,381.6	4,196.6
IA&C
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenue	3,633.2	3,766.3	3,679.7
Total revenue	4,002.2	4,140.6	4,041.8
SC&E
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenue	1,440.9	1,386.2	1,310.5
Total revenue	2,492.3	2,405.6	2,002.3
United States
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenue	6,102.2	6,157.7	5,763.1
Total revenue	7,033.1	7,031.0	6,360.2
United States | MD&E
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenue	2,703.5	2,690.9	2,552.0
Total revenue	2,731.4	2,730.2	2,584.7
United States | IA&C
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenue	2,383.8	2,483.7	2,306.0
Total revenue	2,508.8	2,585.9	2,403.3
United States | SC&E
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenue	1,014.9	983.1	905.1
Total revenue	1,792.9	1,714.9	1,372.2
Total International
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenue	3,298.4	3,291.7	3,344.8
Total revenue	3,856.2	3,896.8	3,880.5
Total International | MD&E
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenue	1,623.0	1,606.0	1,565.7
Total revenue	1,663.4	1,651.4	1,611.9
Total International | IA&C
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenue	1,249.4	1,282.6	1,373.7
Total revenue	1,493.4	1,554.7	1,638.5
Total International | SC&E
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenue	426.0	403.1	405.4
Total revenue	$ 699.4	$ 690.7	$ 630.1