Segment Reporting (Segment Reporting Information) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Revenues:
Revenues	$ 1,025.7	$ 990.5
Gross Profit:
Total Gross Profit	281.0	280.9
Operating Income (Loss):
Total Operating Income	74.8	70.9
Amortization of intangible assets	12.2	12.1
Apex
Revenues:
Revenues	630.4	629.1
Gross Profit:
Total Gross Profit	182.0	184.5
Operating Income (Loss):
Total Operating Income	68.5	67.1
Amortization of intangible assets	5.7	5.3
Oxford
Revenues:
Revenues	137.5	148.7
Gross Profit:
Total Gross Profit	54.0	59.3
Operating Income (Loss):
Total Operating Income	11.3	11.1
Amortization of intangible assets	0.1	0.2
ECS
Revenues:
Revenues	257.8	212.7
Gross Profit:
Total Gross Profit	45.0	37.1
Operating Income (Loss):
Total Operating Income	14.8	11.3
Amortization of intangible assets	6.4	6.6
Assignment | Apex
Revenues:
Revenues	512.8	539.5
Assignment | Oxford
Revenues:
Revenues	117.4	135.5
Consulting | Apex
Revenues:
Revenues	117.6	89.6
Consulting | Oxford
Revenues:
Revenues	20.1	13.2
Firm-fixed-price | ECS
Revenues:
Revenues	60.5	57.0
Time and materials | ECS
Revenues:
Revenues	90.7	72.4
Cost reimbursable | ECS
Revenues:
Revenues	$ 106.6	$ 83.3