Segment Reporting (Segment Reporting Information) (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) segment	Jun. 30, 2016 USD ($)
Revenues:
Revenues	$ 653,313	$ 608,088	$ 1,279,841	$ 1,190,128
Revenue as Percentage of Consolidated Revenue	100.00%	100.00%	100.00%	100.00%
Gross Profit:
Total Gross Profit	$ 212,937	$ 202,086	$ 411,081	$ 389,868
Operating Income (Loss):
Total Operating Income	59,461	50,704	103,069	88,461
Amortization of Intangible Assets	8,299	10,032	$ 16,763	20,176
Reportable segments (in number of segments) | segment			2
Apex
Revenues:
Revenues	502,455	453,723	$ 984,970	886,878
Gross Profit:
Total Gross Profit	150,223	138,165	290,142	264,309
Operating Income (Loss):
Total Operating Income	56,770	50,190	103,663	90,176
Amortization of Intangible Assets	7,262	8,590	14,789	17,180
Oxford
Revenues:
Revenues	150,858	154,365	294,871	303,250
Gross Profit:
Total Gross Profit	62,714	63,921	120,939	125,559
Operating Income (Loss):
Total Operating Income	14,774	13,856	23,437	26,616
Amortization of Intangible Assets	1,037	1,442	1,974	2,996
Corporate Segment [Member]
Operating Income (Loss):
Total Operating Income	(12,083)	(13,342)	(24,031)	(28,331)
Amortization of Intangible Assets	0	0	0	0
Assignment [Member]
Revenues:
Revenues	$ 620,014	$ 574,323	$ 1,214,529	$ 1,123,875
Revenue as Percentage of Consolidated Revenue	94.90%	94.40%	94.90%	94.40%
PermanentPlacement [Member]
Revenues:
Revenues	$ 33,299	$ 33,765	$ 65,312	$ 66,253
Revenue as Percentage of Consolidated Revenue	5.10%	5.60%	5.10%	5.60%