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Segment Reporting Revenue by Customer Type (details) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Revenue from External Customer [Line Items]
Revenues	$ 1,073.8	$ 904.4	$ 2,955.7	$ 2,601.7
Department of Defense and Intelligence Agencies
Revenue from External Customer [Line Items]
Revenues	166.0	172.0	450.8	414.4
Federal Civilian
Revenue from External Customer [Line Items]
Revenues	112.1	96.2	303.5	276.1
Other
Revenue from External Customer [Line Items]
Revenues	20.8	20.4	64.7	50.5
Federal Government Business
Revenue from External Customer [Line Items]
Revenues	$ 298.9	$ 288.6	$ 819.0	$ 741.0

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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