Segment Reporting (Revenue by Geographical Areas) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Revenues from External Customers [Line Items]
Revenues	$ 485,323	$ 434,424	$ 915,368	$ 841,275
Revenue as Percentage of Consolidated Revenue	100.00%	100.00%	100.00%	100.00%
Domestic [Member]
Revenues from External Customers [Line Items]
Revenues	$ 462,989	$ 414,624	$ 873,745	$ 801,577
Revenue as Percentage of Consolidated Revenue	95.40%	95.40%	95.50%	95.30%
Foreign [Member]
Revenues from External Customers [Line Items]
Revenues	$ 22,334	$ 19,800	$ 41,623	$ 39,698
Revenue as Percentage of Consolidated Revenue	4.60%	4.60%	4.50%	4.70%
Assignment [Member]
Revenues from External Customers [Line Items]
Revenues	$ 456,592	$ 412,953	$ 862,733	$ 800,810
Revenue as Percentage of Consolidated Revenue	94.10%	95.10%	94.20%	95.20%
Direct hire and conversion fees [Member]
Revenues from External Customers [Line Items]
Revenues	$ 28,731	$ 21,471	$ 52,635	$ 40,465
Revenue as Percentage of Consolidated Revenue	5.90%	4.90%	5.80%	4.80%