Segment Reporting (Segment Reporting Information) (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2016 USD ($) segment	Mar. 31, 2015 USD ($)
Revenues:
Revenues	$ 582,040	$ 430,045
Revenue as Percentage of Consolidated Revenue	100.00%	100.00%
Gross Profit:
Total Gross Profit	$ 187,782	$ 135,875
Operating Income (Loss):
Total Operating Income	37,757	25,071
Amortization of Intangible Assets	$ 10,144	4,869
Reportable segments (in number of segments) | segment	2
Apex
Revenues:
Revenues	$ 433,155	295,971
Gross Profit:
Total Gross Profit	126,144	80,152
Operating Income (Loss):
Total Operating Income	39,986	22,896
Amortization of Intangible Assets	8,590	3,522
Oxford
Revenues:
Revenues	148,885	134,074
Gross Profit:
Total Gross Profit	61,638	55,723
Operating Income (Loss):
Total Operating Income	12,760	12,713
Amortization of Intangible Assets	1,554	1,347
Corporate Segment [Member]
Operating Income (Loss):
Total Operating Income	(14,989)	(10,538)
Amortization of Intangible Assets	0	0
Assignment [Member]
Revenues:
Revenues	$ 549,552	$ 406,141
Revenue as Percentage of Consolidated Revenue	94.40%	94.40%
PermanentPlacement [Member]
Revenues:
Revenues	$ 32,488	$ 23,904
Revenue as Percentage of Consolidated Revenue	5.60%	5.60%