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Segment Reporting - Segment Reporting Information (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022
Segment Reporting Information [Line Items] |  |  |
Revenues | $ 4,099.7 | $ 4,450.6 | $ 4,581.1
Cost of services | 2,916.0 | 3,170.6 | 3,211.5
Gross Profit | 1,183.7 | 1,280.0 | 1,369.6
Selling, general, and administrative expenses | 821.2 | 844.2 | 895.0
Amortization of intangible assets | 58.1 | 71.7 | 65.1
Operating income | 304.4 | 364.1 | 409.5
Interest expense, net | 64.3 | 66.4 | 45.9
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest | 240.1 | 297.7 | 363.6
Federal Government Business |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenues | 1,231.0 | 1,276.2 | 1,145.4
Operating Segments [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenues | 4,099.7 | 4,450.6 | 4,581.1
Cost of services | 2,916.0 | 3,170.6 | 3,211.5
Gross Profit | 1,183.7 | 1,280.0 | 1,369.6
Other Depreciation and Amortization | 34.6 | 24.6 | 20.9
Other Expenses | 709.3 | 740.4 | 783.4
Selling, general, and administrative expenses | 743.9 | 765.0 | 804.3
Amortization of intangible assets | 58.1 | 71.7 | 65.1
Operating income | 381.7 | 443.3 | 500.2
Operating Segments [Member] | Consulting |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenues | 2,359.2 | 2,371.7 | 2,105.0
Operating Segments [Member] | Assignment |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenues | 1,740.5 | 2,078.9 | 2,476.1
Operating Segments [Member] | Commercial Business |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenues | 2,868.7 | 3,174.4 | 3,435.7
Cost of services | 1,935.8 | 2,156.8 | 2,309.5
Gross Profit | 932.9 | 1,017.6 | 1,126.2
Other Depreciation and Amortization | 29.6 | 20.3 | 16.4
Other Expenses | 588.3 | 618.4 | 667.4
Selling, general, and administrative expenses | 617.9 | 638.7 | 683.8
Amortization of intangible assets | 28.5 | 34.8 | 31.3
Operating income | 286.5 | 344.1 | 411.1
Operating Segments [Member] | Commercial Business | Consulting |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenues | 1,128.2 | 1,095.5 | 959.6
Operating Segments [Member] | Commercial Business | Assignment |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenues | 1,740.5 | 2,078.9 | 2,476.1
Operating Segments [Member] | Federal Government Business |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenues | 1,231.0 | 1,276.2 | 1,145.4
Cost of services | 980.2 | 1,013.8 | 902.0
Gross Profit | 250.8 | 262.4 | 243.4
Other Depreciation and Amortization | 5.0 | 4.3 | 4.5
Other Expenses | 121.0 | 122.0 | 116.0
Selling, general, and administrative expenses | 126.0 | 126.3 | 120.5
Amortization of intangible assets | 29.6 | 36.9 | 33.8
Operating income | 95.2 | 99.2 | 89.1
Operating Segments [Member] | Federal Government Business | Assignment |  |  |
Segment Reporting Information [Line Items] |  |  |
Revenues | 0.0 | 0.0 | 0.0
Corporate Non Segment [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Corporate Expenses | $ 77.3 | $ 79.2 | $ 90.7

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