Disaggregation of Revenue - Schedule of Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Total Revenue	$ 345,558	$ 326,584	$ 1,011,157	$ 973,560
Flex revenue
Disaggregation of Revenue [Line Items]
Total Revenue	333,652	316,102	973,987	939,147
Direct Hire revenue
Disaggregation of Revenue [Line Items]
Total Revenue	11,906	10,482	37,170	34,413
Tech
Disaggregation of Revenue [Line Items]
Total Revenue	271,999	251,079	792,947	737,339
Tech | Flex revenue
Disaggregation of Revenue [Line Items]
Total Revenue	267,304	247,154	777,227	723,159
Tech | Direct Hire revenue
Disaggregation of Revenue [Line Items]
Total Revenue	4,695	3,925	15,720	14,180
FA
Disaggregation of Revenue [Line Items]
Total Revenue	73,559	75,505	218,210	236,221
FA | Flex revenue
Disaggregation of Revenue [Line Items]
Total Revenue	66,348	68,948	196,760	215,988
FA | Direct Hire revenue
Disaggregation of Revenue [Line Items]
Total Revenue	$ 7,211	$ 6,557	$ 21,450	$ 20,233