Segment Reporting (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Segment Reporting Information [Line Items]
Revenue	$ 4,037.1	$ 3,882.6	$ 11,743.1	$ 11,366.9
Salary and service costs	2,921.5	2,796.0	8,600.4	8,288.7
Occupancy and other costs	322.7	325.6	963.2	953.9
Segment cost of services	3,282.8	3,121.6	9,691.0	9,300.4
Selling, general and administrative expenses	163.5	99.5	451.8	295.8
Depreciation and amortization	60.7	61.4	178.4	181.4
Acquisition related costs	60.8		160.6
Repositioning costs	38.6	0.0	127.4	57.8
Total Operating Expenses	3,507.0	3,282.5	10,321.2	9,777.6
Operating Income	530.1	600.1	1,421.9	1,589.3
Interest Expense	60.4	66.4	182.1	182.9
Interest Income	17.2	26.0	68.8	74.0
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	486.9	559.7	1,308.6	1,480.4
Operating Segments | Reportable Segment
Segment Reporting Information [Line Items]
Revenue	4,037.1	3,882.6	11,743.1	11,366.9
Salary and related costs	1,778.5	1,846.9	5,386.8	5,531.1
Third-party service costs	955.6	784.5	2,670.8	2,293.8
Third-party incidental costs	187.4	164.6	542.8	463.8
Salary and service costs	2,921.5	2,796.0	8,600.4	8,288.7
Occupancy and other costs	322.7	325.6	963.2	953.9
Segment cost of services	3,244.2	3,121.6	9,563.6	9,242.6
Selling, general and administrative expenses	102.7	99.5	291.2	295.8
Depreciation and amortization	60.7	61.4	178.4	181.4
Total Operating Expenses	3,407.6	3,282.5	10,033.2	9,719.8
Operating Income	629.5	600.1	1,709.9	1,647.1
Segment Reporting, Reconciling Item
Segment Reporting Information [Line Items]
Acquisition related costs	60.8	0.0	160.6	0.0
Repositioning costs	$ 38.6	$ 0.0	$ 127.4	$ 57.8