Revenue Recognition - Schedule of Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Total Revenue	$ 355,452	$ 341,053	$ 1,060,369	$ 1,015,354
Flex revenue
Disaggregation of Revenue [Line Items]
Total Revenue	339,937	326,335	1,014,859	969,682
Direct Hire revenue
Disaggregation of Revenue [Line Items]
Total Revenue	10,482	12,149	34,413	37,507
Product revenue
Disaggregation of Revenue [Line Items]
Total Revenue	5,033	2,569	11,097	8,165
Tech
Disaggregation of Revenue [Line Items]
Total Revenue	251,079	229,281	737,339	679,695
Tech | Flex revenue
Disaggregation of Revenue [Line Items]
Total Revenue	247,154	224,148	723,159	663,778
Tech | Direct Hire revenue
Disaggregation of Revenue [Line Items]
Total Revenue	3,925	5,133	14,180	15,917
Tech | Product revenue
Disaggregation of Revenue [Line Items]
Total Revenue	0	0	0	0
FA
Disaggregation of Revenue [Line Items]
Total Revenue	75,505	85,225	236,221	260,786
FA | Flex revenue
Disaggregation of Revenue [Line Items]
Total Revenue	68,948	78,209	215,988	239,196
FA | Direct Hire revenue
Disaggregation of Revenue [Line Items]
Total Revenue	6,557	7,016	20,233	21,590
FA | Product revenue
Disaggregation of Revenue [Line Items]
Total Revenue	0	0	0	0
GS
Disaggregation of Revenue [Line Items]
Total Revenue	28,868	26,547	86,809	74,873
GS | Flex revenue
Disaggregation of Revenue [Line Items]
Total Revenue	23,835	23,978	75,712	66,708
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	$ 5,033	$ 2,569	$ 11,097	$ 8,165