Segment Reporting (Segment Reporting Information) (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2017 USD ($) segment	Mar. 31, 2016 USD ($)
Revenues:
Revenues	$ 626,528	$ 582,040
Revenue as Percentage of Consolidated Revenue	100.00%	100.00%
Gross Profit:
Total Gross Profit	$ 198,144	$ 187,782
Operating Income (Loss):
Total Operating Income	43,608	37,757
Amortization of Intangible Assets	$ 8,464	10,144
Reportable segments (in number of segments) | segment	2
Apex
Revenues:
Revenues	$ 482,515	433,155
Gross Profit:
Total Gross Profit	139,919	126,144
Operating Income (Loss):
Total Operating Income	46,893	39,986
Amortization of Intangible Assets	7,527	8,590
Oxford
Revenues:
Revenues	144,013	148,885
Gross Profit:
Total Gross Profit	58,225	61,638
Operating Income (Loss):
Total Operating Income	8,663	12,760
Amortization of Intangible Assets	937	1,554
Corporate Segment [Member]
Operating Income (Loss):
Total Operating Income	(11,948)	(14,989)
Amortization of Intangible Assets	0	0
Assignment [Member]
Revenues:
Revenues	$ 594,515	$ 549,552
Revenue as Percentage of Consolidated Revenue	94.90%	94.40%
PermanentPlacement [Member]
Revenues:
Revenues	$ 32,013	$ 32,488
Revenue as Percentage of Consolidated Revenue	5.10%	5.60%