Segment Reporting - Disaggregated Revenue (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Revenue from External Customer [Line Items]
Revenues	$ 4,581.1	$ 4,009.5	$ 3,502.1
Commercial Business
Revenue from External Customer [Line Items]
Revenues	3,435.7	2,927.1	2,497.9
Commercial Business | Assignment
Revenue from External Customer [Line Items]
Revenues	2,476.1	2,285.9	2,117.0
Commercial Business | Consulting
Revenue from External Customer [Line Items]
Revenues	959.6	641.2	380.9
Federal Government Business
Revenue from External Customer [Line Items]
Revenues	1,145.4	1,082.4	1,004.2
Federal Government Business | Firm-fixed-price
Revenue from External Customer [Line Items]
Revenues	331.6	295.6	272.0
Federal Government Business | Time and materials
Revenue from External Customer [Line Items]
Revenues	456.3	399.0	322.6
Federal Government Business | Cost reimbursable
Revenue from External Customer [Line Items]
Revenues	$ 357.5	$ 387.8	$ 409.6