Revenue Recognition - Schedule of Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation of Revenue [Line Items]
Total Revenue	$ 358,624	$ 340,309	$ 704,917	$ 674,301
Flex revenue
Disaggregation of Revenue [Line Items]
Total Revenue	342,105	324,015	674,922	643,347
Direct Hire revenue
Disaggregation of Revenue [Line Items]
Total Revenue	12,536	13,853	23,931	25,358
Product revenue
Disaggregation of Revenue [Line Items]
Total Revenue	3,983	2,441	6,064	5,596
Tech
Disaggregation of Revenue [Line Items]
Total Revenue	249,763	228,369	486,260	450,414
Tech | Flex revenue
Disaggregation of Revenue [Line Items]
Total Revenue	244,509	222,744	476,005	439,630
Tech | Direct Hire revenue
Disaggregation of Revenue [Line Items]
Total Revenue	5,254	5,625	10,255	10,784
Tech | Product revenue
Disaggregation of Revenue [Line Items]
Total Revenue	0	0	0	0
FA
Disaggregation of Revenue [Line Items]
Total Revenue	79,772	88,266	160,716	175,561
FA | Flex revenue
Disaggregation of Revenue [Line Items]
Total Revenue	72,490	80,038	147,040	160,987
FA | Direct Hire revenue
Disaggregation of Revenue [Line Items]
Total Revenue	7,282	8,228	13,676	14,574
FA | Product revenue
Disaggregation of Revenue [Line Items]
Total Revenue	0	0	0	0
GS
Disaggregation of Revenue [Line Items]
Total Revenue	29,089	23,674	57,941	48,326
GS | Flex revenue
Disaggregation of Revenue [Line Items]
Total Revenue	25,106	21,233	51,877	42,730
GS | Direct Hire revenue
Disaggregation of Revenue [Line Items]
Total Revenue	0	0	0	0
GS | Product revenue
Disaggregation of Revenue [Line Items]
Total Revenue	$ 3,983	$ 2,441	$ 6,064	$ 5,596