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Segment Reporting - Revenue by Customer Type (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Revenue from External Customer [Line Items]
Revenues	$ 1,011.4	$ 1,031.0	$ 3,000.3	$ 3,114.7
Federal Government
Revenue from External Customer [Line Items]
Revenues	300.1	312.2	908.7	938.7
Federal Government | Firm-fixed-price
Revenue from External Customer [Line Items]
Revenues	84.2	92.6	260.2	273.8
Federal Government | T&M
Revenue from External Customer [Line Items]
Revenues	117.0	128.0	371.2	395.4
Federal Government | Cost reimbursable
Revenue from External Customer [Line Items]
Revenues	98.9	91.6	277.3	269.5
Other Federal [Member]
Revenue from External Customer [Line Items]
Revenues	31.6	31.4	93.9	93.0
Federal Civilian
Revenue from External Customer [Line Items]
Revenues	80.7	72.1	242.1	209.7
Department of Homeland Security [Member]
Revenue from External Customer [Line Items]
Revenues	57.1	67.2	176.9	202.8
Department of Defense and Intelligence Agencies
Revenue from External Customer [Line Items]
Revenues	$ 130.7	$ 141.5	$ 395.8	$ 433.2

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