Segment Reporting (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Segment Reporting Information [Line Items]
Revenue	$ 3,690.4	$ 3,630.5
Salary and service costs	2,746.3	2,692.6
Occupancy and other costs	314.6	314.1
Segment cost of services	3,060.9	3,006.7
Selling, general and administrative expenses	117.9	85.3
Depreciation and amortization	59.0	59.6
Acquisition related costs	33.8
Total Operating Expenses	3,237.8	3,151.6
Operating Income	452.6	478.9
Interest Expense	59.1	53.8
Interest Income	29.7	27.0
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	423.2	452.1
Operating Segments | Reportable Segment
Segment Reporting Information [Line Items]
Revenue	3,690.4	3,630.5
Salary and related costs	1,780.5	1,847.3
Third-party service costs	796.8	698.2
Third-party incidental costs	169.0	147.1
Salary and service costs	2,746.3	2,692.6
Occupancy and other costs	314.6	314.1
Segment cost of services	3,060.9	3,006.7
Selling, general and administrative expenses	84.1	85.3
Depreciation and amortization	59.0	59.6
Total Operating Expenses	3,204.0	3,151.6
Operating Income	486.4	478.9
Segment Reporting, Reconciling Item
Segment Reporting Information [Line Items]
Acquisition related costs	$ 33.8	$ 0.0