Segment Reporting (Segment Reporting Information) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Revenues:
Revenues	$ 1,130.9	$ 1,141.8	$ 2,259.7	$ 2,232.8
Gross Profit:
Total Gross Profit	326.3	344.0	652.7	670.6
Operating Income (Loss):
Total Operating Income	97.9	110.1	182.1	210.7
Depreciation	7.0	6.1	13.8	12.3
Amortization of intangible assets	17.9	13.5	36.0	27.4
Commercial Business
Revenues:
Revenues	811.3	850.6	1,643.4	1,683.5
Gross Profit:
Total Gross Profit	260.9	281.7	523.3	554.3
Operating Income (Loss):
Total Operating Income	91.7	106.0	175.6	211.0
Depreciation	4.9	3.5	9.5	7.1
Amortization of intangible assets	8.7	5.6	17.5	11.3
Federal Government Business
Revenues:
Revenues	319.6	291.2	616.3	549.3
Gross Profit:
Total Gross Profit	65.4	62.3	129.4	116.3
Operating Income (Loss):
Total Operating Income	25.1	25.7	47.7	42.7
Depreciation	1.5	1.4	2.9	2.9
Amortization of intangible assets	9.2	7.9	18.5	16.2
Assignment | Commercial Business
Revenues:
Revenues	530.2	628.4	1,090.6	1,256.6
Consulting | Commercial Business
Revenues:
Revenues	281.1	222.2	552.8	426.9
Firm-fixed-price | Federal Government Business
Revenues:
Revenues	97.8	74.3	190.6	151.2
Time and materials | Federal Government Business
Revenues:
Revenues	129.2	117.8	251.1	224.8
Cost reimbursable | Federal Government Business
Revenues:
Revenues	$ 92.6	$ 99.1	$ 174.6	$ 173.3