Segment Reporting (Segment Reporting Information) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Revenues:
Revenues	$ 1,116.8	$ 1,197.9	$ 3,376.5	$ 3,430.7
Gross Profit:
Total Gross Profit	322.4	358.9	975.1	1,029.5
Operating Income (Loss):
Total Operating Income	98.6	108.4	280.7	319.1
Depreciation	7.0	7.2	20.8	19.5
Amortization of intangible assets	17.8	17.9	53.8	45.3
Commercial Business
Revenues:
Revenues	782.4	900.0	2,425.8	2,583.5
Gross Profit:
Total Gross Profit	254.2	297.8	777.5	852.1
Operating Income (Loss):
Total Operating Income	91.4	108.7	267.0	319.6
Depreciation	5.0	5.0	14.5	12.0
Amortization of intangible assets	8.6	9.9	26.1	21.1
Federal Government Business
Revenues:
Revenues	334.4	297.9	950.7	847.2
Gross Profit:
Total Gross Profit	68.2	61.1	197.6	177.4
Operating Income (Loss):
Total Operating Income	27.2	23.0	74.9	66.4
Depreciation	1.4	1.3	4.3	4.2
Amortization of intangible assets	9.2	8.0	27.7	24.2
Assignment | Commercial Business
Revenues:
Revenues	508.2	631.4	1,598.8	1,888.0
Consulting | Commercial Business
Revenues:
Revenues	274.2	268.6	827.0	695.5
Firm-fixed-price | Federal Government Business
Revenues:
Revenues	107.3	91.7	297.9	242.9
Time and materials | Federal Government Business
Revenues:
Revenues	126.8	113.7	377.9	338.5
Cost reimbursable | Federal Government Business
Revenues:
Revenues	$ 100.3	$ 92.5	$ 274.9	$ 265.8