Segment Reporting (Segment Reporting Information) (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) segment	Sep. 30, 2015 USD ($)
Revenues:
Revenues	$ 629,401	$ 572,123	$ 1,819,529	$ 1,487,491
Revenue as Percentage of Consolidated Revenue	100.00%	100.00%	100.00%	100.00%
Gross Profit:
Total Gross Profit	$ 207,120	$ 191,404	$ 596,988	$ 485,813
Operating Income (Loss):
Total Operating Income	55,410	51,465	143,871	109,246
Amortization of Intangible Assets	9,742	11,325	$ 29,918	23,151
Reportable segments (in number of segments) | segment			2
Apex
Revenues:
Revenues	473,546	422,632	$ 1,360,424	1,059,083
Gross Profit:
Total Gross Profit	143,681	129,232	407,990	307,658
Operating Income (Loss):
Total Operating Income	53,982	47,988	144,158	105,149
Amortization of Intangible Assets	8,590	9,732	25,770	18,638
Oxford
Revenues:
Revenues	155,855	149,491	459,105	428,408
Gross Profit:
Total Gross Profit	63,439	62,172	188,998	178,155
Operating Income (Loss):
Total Operating Income	14,429	16,602	41,045	45,140
Amortization of Intangible Assets	1,152	1,593	4,148	4,513
Corporate Segment [Member]
Operating Income (Loss):
Total Operating Income	(13,001)	(13,125)	(41,332)	(41,043)
Amortization of Intangible Assets	0	0	0	0
Assignment [Member]
Revenues:
Revenues	$ 596,437	$ 539,383	$ 1,720,312	$ 1,402,116
Revenue as Percentage of Consolidated Revenue	94.80%	94.30%	94.50%	94.30%
PermanentPlacement [Member]
Revenues:
Revenues	$ 32,964	$ 32,740	$ 99,217	$ 85,375
Revenue as Percentage of Consolidated Revenue	5.20%	5.70%	5.50%	5.70%