Segment Reporting (Segment Reporting Information) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Revenues:
Revenues	$ 1,034.7	$ 1,130.9	$ 2,083.7	$ 2,259.7
Gross Profit:
Total Gross Profit	301.1	326.3	597.3	652.7
Operating Income (Loss):
Total Operating Income	80.4	97.9	151.3	182.1
Depreciation	9.4	7.0	18.8	13.8
Amortization of intangible assets	15.1	17.9	30.2	36.0
Cloud Based Application
Operating Income (Loss):
Amortization of intangible assets	1.2		2.5
Commercial
Revenues:
Revenues	725.7	811.3	1,457.2	1,643.4
Gross Profit:
Total Gross Profit	237.3	260.9	471.1	523.3
Operating Income (Loss):
Total Operating Income	74.7	91.7	143.5	175.6
Depreciation	7.3	4.9	14.5	9.5
Amortization of intangible assets	7.7	8.7	15.4	17.5
Commercial | Assignment
Revenues:
Revenues	444.2	530.2	898.7	1,090.6
Commercial | Consulting
Revenues:
Revenues	281.5	281.1	558.5	552.8
Federal Government
Revenues:
Revenues	309.0	319.6	626.5	616.3
Gross Profit:
Total Gross Profit	63.8	65.4	126.2	129.4
Operating Income (Loss):
Total Operating Income	24.7	25.1	47.7	47.7
Depreciation	1.7	1.5	3.4	2.9
Amortization of intangible assets	7.4	9.2	14.8	18.5
Federal Government | Firm-fixed-price
Revenues:
Revenues	92.0	97.8	181.2	190.6
Federal Government | Time and materials
Revenues:
Revenues	136.2	129.2	267.4	251.1
Federal Government | Cost reimbursable
Revenues:
Revenues	$ 80.8	$ 92.6	$ 177.9	$ 174.6