Segment Reporting (Segment Reporting Information) (Details) $ in Thousands		3 Months Ended
		Mar. 31, 2018 USD ($) segment	Mar. 31, 2017 USD ($)
Revenues:
Revenues		$ 685,173	$ 626,528
Revenue as Percentage of Consolidated Revenue		100.00%	100.00%
Gross Profit:
Total Gross Profit		$ 217,737	$ 198,144
Operating Income (Loss):
Total Operating Income		45,692	43,608
Amortization of Intangible Assets		$ 7,601	8,464
Reportable segments (in number of segments) | segment		2
Apex
Revenues:
Revenues		$ 538,504	482,515
Gross Profit:
Total Gross Profit		158,648	139,919
Operating Income (Loss):
Total Operating Income		56,264	46,893
Amortization of Intangible Assets		6,546	7,527
Oxford
Revenues:
Revenues		146,669	144,013
Gross Profit:
Total Gross Profit		59,089	58,225
Operating Income (Loss):
Total Operating Income		9,826	8,663
Amortization of Intangible Assets		1,055	937
Corporate Segment [Member]
Operating Income (Loss):
Total Operating Income	[1]	(20,398)	(11,948)
Amortization of Intangible Assets		0	0
Assignment [Member]
Revenues:
Revenues		650,256	594,515
Assignment [Member] | Apex
Revenues:
Revenues		524,854	471,266
Assignment [Member] | Oxford
Revenues:
Revenues		125,402	123,249
PermanentPlacement [Member]
Revenues:
Revenues		34,917	32,013
PermanentPlacement [Member] | Apex
Revenues:
Revenues		13,650	11,249
PermanentPlacement [Member] | Oxford
Revenues:
Revenues		$ 21,267	$ 20,764

[1] Corporate expenses primarily consist 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. Corporate expenses for the three months ended March 31, 2018 included $9.8 million of one-time expenses related to the acquisition of ECS Federal, LLC, which closed on April 2, 2018, see "Note 12. Subsequent Events."