Segment Reporting (Segment Reporting Information) (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) segment	Sep. 30, 2016 USD ($)
Revenues:
Revenues	$ 667,048	$ 629,401	$ 1,946,889	$ 1,819,529
Revenue as Percentage of Consolidated Revenue	100.00%	100.00%	100.00%	100.00%
Gross Profit:
Total Gross Profit	$ 218,315	$ 207,120	$ 629,396	$ 596,988
Operating Income (Loss):
Total Operating Income	60,870	55,410	163,939	143,871
Amortization of Intangible Assets	8,248	9,742	$ 25,011	29,918
Reportable segments (in number of segments) | segment			2
Apex
Revenues:
Revenues	517,492	473,546	$ 1,502,462	1,360,424
Gross Profit:
Total Gross Profit	155,783	143,681	445,925	407,990
Operating Income (Loss):
Total Operating Income	59,016	53,982	162,679	144,158
Amortization of Intangible Assets	7,194	8,590	21,983	25,770
Oxford
Revenues:
Revenues	149,556	155,855	444,427	459,105
Gross Profit:
Total Gross Profit	62,532	63,439	183,471	188,998
Operating Income (Loss):
Total Operating Income	16,088	14,429	39,525	41,045
Amortization of Intangible Assets	1,054	1,152	3,028	4,148
Corporate Segment [Member]
Operating Income (Loss):
Total Operating Income	(14,234)	(13,001)	(38,265)	(41,332)
Amortization of Intangible Assets	0	0	0	0
Assignment [Member]
Revenues:
Revenues	$ 634,339	$ 596,437	$ 1,848,868	$ 1,720,312
Revenue as Percentage of Consolidated Revenue	95.10%	94.80%	95.00%	94.50%
PermanentPlacement [Member]
Revenues:
Revenues	$ 32,709	$ 32,964	$ 98,021	$ 99,217
Revenue as Percentage of Consolidated Revenue	4.90%	5.20%	5.00%	5.50%