Segment Reporting (Segment Reporting Information) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Revenues:
Revenues	$ 1,141.8	$ 974.9	$ 2,232.8	$ 1,881.9
Gross Profit:
Total Gross Profit	344.0	276.3	670.6	520.0
Operating Income (Loss):
Total Operating Income	110.1	87.9	210.7	155.3
Depreciation	6.1	7.1	12.3	14.6
Amortization of intangible assets	13.5	12.0	27.4	24.0
Commercial Business
Revenues:
Revenues	850.6	712.5	1,683.5	1,361.7
Gross Profit:
Total Gross Profit	281.7	228.3	554.3	426.9
Operating Income (Loss):
Total Operating Income	106.0	89.6	211.0	160.1
Depreciation	3.5	3.4	7.1	7.0
Amortization of intangible assets	5.6	5.6	11.3	11.3
Federal Government Business
Revenues:
Revenues	291.2	262.4	549.3	520.2
Gross Profit:
Total Gross Profit	62.3	48.0	116.3	93.1
Operating Income (Loss):
Total Operating Income	25.7	18.7	42.7	33.4
Depreciation	1.4	2.2	2.9	4.6
Amortization of intangible assets	7.9	6.4	16.2	12.7
Assignment | Commercial Business
Revenues:
Revenues	628.4	568.1	1,256.6	1,099.8
Consulting | Commercial Business
Revenues:
Revenues	222.2	144.4	426.9	261.9
Firm-fixed-price | Federal Government Business
Revenues:
Revenues	74.3	62.0	151.2	122.6
Time and materials | Federal Government Business
Revenues:
Revenues	117.8	100.2	224.8	190.9
Cost reimbursable | Federal Government Business
Revenues:
Revenues	$ 99.1	$ 100.2	$ 173.3	$ 206.7