Segment Reporting (Segment Reporting Information) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Revenues:
Revenues	$ 1,091.0	$ 907.0
Gross Profit:
Total Gross Profit	326.6	243.7
Operating Income (Loss):
Total Operating Income	100.6	67.4
Depreciation	6.2	7.5
Amortization of intangible assets	13.9	12.0
Commercial Business
Revenues:
Revenues	832.9	649.2
Gross Profit:
Total Gross Profit	272.6	198.7
Operating Income (Loss):
Total Operating Income	104.9	70.5
Depreciation	3.5	3.5
Amortization of intangible assets	5.6	5.7
Federal Government Business
Revenues:
Revenues	258.1	257.8
Gross Profit:
Total Gross Profit	54.0	45.0
Operating Income (Loss):
Total Operating Income	17.3	14.8
Depreciation	1.5	2.4
Amortization of intangible assets	8.3	6.4
Assignment | Commercial Business
Revenues:
Revenues	628.2	531.7
Consulting | Commercial Business
Revenues:
Revenues	204.7	117.5
Firm-fixed-price | Federal Government Business
Revenues:
Revenues	76.9	60.5
Time and materials | Federal Government Business
Revenues:
Revenues	107.0	90.7
Cost reimbursable | Federal Government Business
Revenues:
Revenues	$ 74.2	$ 106.6