Segment Reporting (Segment Reporting Information) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Revenues:
Revenues	$ 1,128.8	$ 1,091.0
Gross Profit:
Total Gross Profit	326.4	326.6
Operating Income (Loss):
Total Operating Income	84.2	100.6
Depreciation	6.8	6.2
Amortization of intangible assets	18.1	13.9
Commercial Business
Revenues:
Revenues	832.1	832.9
Gross Profit:
Total Gross Profit	262.4	272.6
Operating Income (Loss):
Total Operating Income	83.9	104.9
Depreciation	4.6	3.5
Amortization of intangible assets	8.8	5.6
Federal Government Business
Revenues:
Revenues	296.7	258.1
Gross Profit:
Total Gross Profit	64.0	54.0
Operating Income (Loss):
Total Operating Income	22.6	17.3
Depreciation	1.4	1.5
Amortization of intangible assets	9.3	8.3
Assignment | Commercial Business
Revenues:
Revenues	560.4	628.2
Consulting | Commercial Business
Revenues:
Revenues	271.7	204.7
Firm-fixed-price | Federal Government Business
Revenues:
Revenues	92.8	76.9
Time and materials | Federal Government Business
Revenues:
Revenues	121.9	107.0
Cost reimbursable | Federal Government Business
Revenues:
Revenues	$ 82.0	$ 74.2