Segment Reporting - Disaggregated Revenue (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Revenue from External Customer [Line Items]
Revenues	$ 4,450.6	$ 4,581.1	$ 4,009.5
Commercial Business
Revenue from External Customer [Line Items]
Revenues	3,174.4	3,435.7	2,927.1
Commercial Business | Assignment
Revenue from External Customer [Line Items]
Revenues	2,078.9	2,476.1	2,285.9
Commercial Business | Consulting
Revenue from External Customer [Line Items]
Revenues	1,095.5	959.6	641.2
Federal Government Business
Revenue from External Customer [Line Items]
Revenues	1,276.2	1,145.4	1,082.4
Federal Government Business | Firm-fixed-price
Revenue from External Customer [Line Items]
Revenues	386.7	331.6	295.6
Federal Government Business | Time and materials
Revenue from External Customer [Line Items]
Revenues	504.9	456.3	399.0
Federal Government Business | Cost reimbursable
Revenue from External Customer [Line Items]
Revenues	$ 384.6	$ 357.5	$ 387.8