Segment Reporting (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Segment Reporting Information [Line Items]
Revenue	$ 15,689.1	$ 14,692.2	$ 14,289.1
Salary and service costs	11,432.5	10,701.2	10,325.9
Occupancy and other costs	1,274.4	1,168.8	1,168.6
Segment cost of services	12,764.7	11,982.7	11,607.9
Selling, general and administrative expenses	408.1	393.7	378.5
Depreciation and amortization	241.7	211.1	219.4
Operating Expenses	13,414.5	12,587.5	12,205.8
Operating Income	2,274.6	2,104.7	2,083.3
Real estate and other repositioning costs	57.8	191.5	0.0
Charges arising from the effects of the war in Ukraine	0.0	0.0	113.4
Gain on disposition of subsidiary	0.0	(78.8)	0.0
Interest Expense	247.9	218.5	208.6
Interest Income	100.9	106.7	70.7
Income Before Income Taxes and Income From Equity Method Investments	2,127.6	1,992.9	1,945.4
Operating Segments [Member] | Reportable Segment [Member]
Segment Reporting Information [Line Items]
Revenue	15,689.1	14,692.2	14,289.1
Salary and related costs	7,441.4	7,212.8	7,197.9
Third-party service costs	3,348.6	2,917.9	2,585.5
Third-party incidental costs	642.5	570.5	542.5
Salary and service costs	11,432.5	10,701.2	10,325.9
Occupancy and other costs	1,274.4	1,168.8	1,168.6
Segment cost of services	12,706.9	11,870.0	11,494.5
Selling, general and administrative expenses	408.1	393.7	378.5
Depreciation and amortization	241.7	211.1	219.4
Operating Expenses	13,356.7	12,474.8	12,092.4
Operating Income	$ 2,332.4	$ 2,217.4	$ 2,196.7