Revenue Income Statement (Details) - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenue	$ 9,187.6	$ 9,400.6	$ 9,449.4
Billable expenses	1,504.1	1,488.7	1,478.4
Total revenue	10,691.7	10,889.3	10,927.8
OPERATING EXPENSES:
Salaries and related expenses	6,024.8	6,243.9	6,258.3
Office and other direct expenses	1,343.1	1,342.5	1,346.4
Billable expense	1,504.1	1,488.7	1,478.4
Cost of services	8,872.0	9,075.1	9,083.1
Selling, General and Administrative Expense	130.5	67.2	87.1
Depreciation and amortization	258.9	264.3	274.0
Total operating expenses	9,488.5	9,406.7	9,546.6
Operating Income (Loss)	1,203.2	1,482.6	1,381.2
EXPENSES AND OTHER INCOME:
Interest Expense, Operating and Nonoperating	(229.9)	(225.6)	(167.9)
Interest income	151.7	140.8	56.6
Other (expense) income, net	(75.9)	10.2	(1.0)
Total (expenses) and other income	(154.1)	(74.6)	(112.3)
Income before income taxes	1,049.1	1,408.0	1,268.9
Provision for income taxes	333.9	291.2	318.4
Income of consolidated companies	715.2	1,116.8	950.5
Equity in net income of unconsolidated affiliates	0.5	1.3	5.6
NET INCOME	715.7	1,118.1	956.1
Net Income Attributable to Parent	$ 689.5	$ 1,098.4	$ 938.0
Earnings per share, Basic	$ 1.84	$ 2.86	$ 2.40
Earnings per share, Diluted	$ 1.83	$ 2.85	$ 2.37
Weighted-average number of common shares outstanding, Basic	375.2	384.1	391.5
Weighted-average number of common shares outstanding, Diluted	377.7	385.9	395.1
MD&E
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenue	$ 4,213.8	$ 4,348.6	$ 4,315.7
Total revenue	4,274.3	4,417.4	4,401.1
OPERATING EXPENSES:
Office and other direct expenses	751.8	739.3	751.2
IA&C
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenue	3,544.4	3,611.1	3,747.5
Total revenue	3,907.2	3,979.6	4,121.1
OPERATING EXPENSES:
Office and other direct expenses	420.4	438.0	427.9
SC&E
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenue	1,429.4	1,440.9	1,386.2
Total revenue	2,510.2	2,492.3	2,405.6
OPERATING EXPENSES:
Office and other direct expenses	170.9	165.2	167.3
United States
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenue	5,940.3	6,102.2	6,157.7
Total revenue	6,837.4	7,033.1	7,031.0
United States | MD&E
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenue	2,644.6	2,709.7	2,696.1
Total revenue	2,662.4	2,737.7	2,735.5
United States | IA&C
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenue	2,283.3	2,377.6	2,478.5
Total revenue	2,392.5	2,502.5	2,580.6
United States | SC&E
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenue	1,012.4	1,014.9	983.1
Total revenue	1,782.5	1,792.9	1,714.9
Total International
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenue	3,247.3	3,298.4	3,291.7
Total revenue	3,854.3	3,856.2	3,896.8
Total International | MD&E
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenue	1,569.2	1,638.9	1,619.6
Total revenue	1,611.9	1,679.7	1,665.6
Total International | IA&C
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenue	1,261.1	1,233.5	1,269.0
Total revenue	1,514.7	1,477.1	1,540.5
Total International | SC&E
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenue	417.0	426.0	403.1
Total revenue	$ 727.7	$ 699.4	$ 690.7