Segment Reporting (Segment Reporting Information) (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Revenues:
Revenues		$ 990.5	$ 923.7
Gross Profit:
Total Gross Profit		280.9	263.9
Operating Income (Loss):
Total Operating Income		70.9	62.7
Amortization of Intangible Assets		12.1	13.8
Apex
Revenues:
Revenues		629.1	606.1
Gross Profit:
Total Gross Profit		184.5	175.4
Operating Income (Loss):
Total Operating Income		67.1	61.0
Amortization of Intangible Assets		5.3	6.0
Oxford
Revenues:
Revenues		148.7	149.6
Gross Profit:
Total Gross Profit		59.3	58.9
Operating Income (Loss):
Total Operating Income		11.1	11.7
Amortization of Intangible Assets		0.2	1.0
ECS
Revenues:
Revenues		212.7	168.0
Gross Profit:
Total Gross Profit		37.1	29.6
Operating Income (Loss):
Total Operating Income		11.3	6.9
Amortization of Intangible Assets		6.6	6.8
Corporate
Operating Income (Loss):
Total Operating Income	[1]	(18.6)	(16.9)
Assignment | Apex
Revenues:
Revenues		615.9	592.2
Assignment | Oxford
Revenues:
Revenues		128.1	129.4
Permanent Placement | Apex
Revenues:
Revenues		13.2	13.9
Permanent Placement | Oxford
Revenues:
Revenues		20.6	20.2
Fixed-price Contract | ECS
Revenues:
Revenues		57.0	43.3
Time and materials Contract | ECS
Revenues:
Revenues		72.4	61.4
Cost-plus-fixed-fee Contract | ECS
Revenues:
Revenues		$ 83.3	$ 63.3

[1] Parent company operating expenses consisting of consolidated stock-based compensation expense, compensation for corporate employees, acquisition, integration and strategic planning expenses, public company expenses and depreciation expense for corporate assets.