Segment Reporting - Segment Reporting Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Segment Reporting Information [Line Items]
Revenues	$ 3,980.4	$ 4,099.7	$ 4,450.6
Cost of services	2,831.3	2,916.0	3,170.6
Gross Profit	1,149.1	1,183.7	1,280.0
Selling, general, and administrative expenses	854.0	821.2	844.2
Amortization of intangible assets	64.8	58.1	71.7
Operating income	230.3	304.4	364.1
Interest expense, net	67.7	64.3	66.4
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	162.6	240.1	297.7
Provision for income taxes	49.1	64.9	78.4
Net income	113.5	175.2	219.3
Federal Government Business
Segment Reporting Information [Line Items]
Revenues	1,190.2	1,231.0	1,276.2
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues	3,980.4	4,099.7	4,450.6
Cost of services	2,831.3	2,916.0	3,170.6
Gross Profit	1,149.1	1,183.7	1,280.0
Other Depreciation and Amortization	45.2	34.6	24.6
Other Expenses	697.7	709.3	740.4
Selling, general, and administrative expenses	742.9	743.9	765.0
Amortization of intangible assets	64.8	58.1	71.7
Operating income	341.4	381.7	443.3
Operating Segments [Member] | Consulting
Segment Reporting Information [Line Items]
Revenues	2,480.3	2,359.2	2,371.7
Operating Segments [Member] | Assignment
Segment Reporting Information [Line Items]
Revenues	1,500.1	1,740.5	2,078.9
Operating Segments [Member] | Commercial Business
Segment Reporting Information [Line Items]
Revenues	2,790.2	2,868.7	3,174.4
Cost of services	1,875.8	1,935.8	2,156.8
Gross Profit	914.4	932.9	1,017.6
Other Depreciation and Amortization	39.3	29.6	20.3
Other Expenses	587.6	588.3	618.4
Selling, general, and administrative expenses	626.9	617.9	638.7
Amortization of intangible assets	39.4	28.5	34.8
Operating income	248.1	286.5	344.1
Operating Segments [Member] | Commercial Business | Consulting
Segment Reporting Information [Line Items]
Revenues	1,290.1	1,128.2	1,095.5
Operating Segments [Member] | Commercial Business | Assignment
Segment Reporting Information [Line Items]
Revenues	1,500.1	1,740.5	2,078.9
Operating Segments [Member] | Federal Government Business
Segment Reporting Information [Line Items]
Revenues	1,190.2	1,231.0	1,276.2
Cost of services	955.5	980.2	1,013.8
Gross Profit	234.7	250.8	262.4
Other Depreciation and Amortization	5.9	5.0	4.3
Other Expenses	110.1	121.0	122.0
Selling, general, and administrative expenses	116.0	126.0	126.3
Amortization of intangible assets	25.4	29.6	36.9
Operating income	93.3	95.2	99.2
Operating Segments [Member] | Federal Government Business | Consulting
Segment Reporting Information [Line Items]
Revenues	1,190.2	1,231.0	1,276.2
Corporate Non Segment [Member]
Segment Reporting Information [Line Items]
Corporate Expenses	$ 111.1	$ 77.3	$ 79.2