Disaggregation of Revenue - Schedule of Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Total Revenue	$ 338,861	$ 329,535	$ 665,599	$ 646,976
Flex revenue
Disaggregation of Revenue [Line Items]
Total Revenue	325,354	316,999	640,335	623,045
Direct Hire revenue
Disaggregation of Revenue [Line Items]
Total Revenue	13,507	12,536	25,264	23,931
Tech
Disaggregation of Revenue [Line Items]
Total Revenue	265,305	249,763	520,948	486,260
Tech | Flex revenue
Disaggregation of Revenue [Line Items]
Total Revenue	259,707	244,509	509,923	476,005
Tech | Direct Hire revenue
Disaggregation of Revenue [Line Items]
Total Revenue	5,598	5,254	11,025	10,255
FA
Disaggregation of Revenue [Line Items]
Total Revenue	73,556	79,772	144,651	160,716
FA | Flex revenue
Disaggregation of Revenue [Line Items]
Total Revenue	65,647	72,490	130,412	147,040
FA | Direct Hire revenue
Disaggregation of Revenue [Line Items]
Total Revenue	$ 7,909	$ 7,282	$ 14,239	$ 13,676