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Segment Reporting - Disaggregated Revenue (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Revenue from External Customer [Line Items]
Revenues	$ 1,053.8	$ 1,073.8	$ 974.9	$ 907.0	$ 900.4	$ 904.4	$ 831.9	$ 865.4	$ 4,009.5	$ 3,502.1	$ 3,415.6
Commercial
Revenue from External Customer [Line Items]
Revenues									2,927.1	2,497.9	2,617.4
Commercial | Assignment
Revenue from External Customer [Line Items]
Revenues									2,285.9	2,117.0	2,288.3
Commercial | Consulting
Revenue from External Customer [Line Items]
Revenues									641.2	380.9	329.1
Federal Government
Revenue from External Customer [Line Items]
Revenues									1,082.4	1,004.2	798.2
Federal Government | Firm-fixed-price
Revenue from External Customer [Line Items]
Revenues									295.6	272.0	214.0
Federal Government | Time and materials
Revenue from External Customer [Line Items]
Revenues									399.0	322.6	267.8
Federal Government | Cost reimbursable
Revenue from External Customer [Line Items]
Revenues									$ 387.8	$ 409.6	$ 316.4

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