Revenue Income Statement (Details) - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenue	$ 9,107.9	$ 8,064.5	$ 8,625.1
Billable expenses	1,132.8	996.5	1,596.2
Total revenue	10,240.7	9,061.0	10,221.3
OPERATING EXPENSES:
Salaries and related expenses	5,975.4	5,345.0	5,568.8
Office and other direct expenses	1,279.6	1,367.9	1,564.1
Billable expense	1,132.8	996.5	1,596.2
Cost of services	8,387.8	7,709.4	8,729.1
Selling, General and Administrative Expense	122.3	58.8	93.8
Depreciation and amortization	283.8	290.6	278.5
Total operating expenses	8,804.5	8,472.6	9,135.3
Operating Income (Loss)	1,436.2	588.4	1,086.0
EXPENSES AND OTHER INCOME:
Interest Expense	(173.1)	(192.2)	(199.3)
Interest income	29.7	29.5	34.5
Other expense, net	(70.7)	(64.4)	(42.9)
Total (expenses) and other income	(214.1)	(227.1)	(207.7)
Income before income taxes	1,222.1	361.3	878.3
Provision for income taxes	251.8	8.0	204.8
Income of consolidated companies	970.3	353.3	673.5
Equity in net income of unconsolidated affiliates	2.5	0.9	0.4
NET INCOME	972.8	354.2	673.9
Net Income Attributable to Parent	$ 952.8	$ 351.1	$ 656.0
Earnings per share, Basic	$ 2.42	$ 0.90	$ 1.70
Earnings per share, Diluted	$ 2.39	$ 0.89	$ 1.68
Weighted-average number of common shares outstanding, Basic	393.0	389.4	386.1
Weighted-average number of common shares outstanding, Diluted	398.4	393.2	391.2