Revenue - Schedule of Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Revenue	$ 357,984	$ 355,452	$ 358,624	$ 346,293	$ 342,586	$ 341,053	$ 340,309	$ 333,992	$ 1,418,353	$ 1,357,940	$ 1,319,706
Flex revenue
Disaggregation of Revenue [Line Items]
Revenue									1,356,463	1,298,210	1,253,106
Direct Hire revenue
Disaggregation of Revenue [Line Items]
Revenue									45,688	47,677	50,399
Product revenue
Disaggregation of Revenue [Line Items]
Revenue									16,202	12,053	16,201
Tech
Disaggregation of Revenue [Line Items]
Revenue									990,089	907,511	883,477
Tech | Flex revenue
Disaggregation of Revenue [Line Items]
Revenue									971,310	887,675	863,434
Tech | Direct Hire revenue
Disaggregation of Revenue [Line Items]
Revenue									18,779	19,836	20,043
Tech | Product revenue
Disaggregation of Revenue [Line Items]
Revenue									0	0	0
FA
Disaggregation of Revenue [Line Items]
Revenue									313,848	346,135	337,601
FA | Flex revenue
Disaggregation of Revenue [Line Items]
Revenue									286,939	318,294	307,245
FA | Direct Hire revenue
Disaggregation of Revenue [Line Items]
Revenue									26,909	27,841	30,356
FA | Product revenue
Disaggregation of Revenue [Line Items]
Revenue									0	0	0
GS
Disaggregation of Revenue [Line Items]
Revenue									114,416	104,294	98,628
GS | Flex revenue
Disaggregation of Revenue [Line Items]
Revenue									98,214	92,241	82,427
GS | Direct Hire revenue
Disaggregation of Revenue [Line Items]
Revenue									0	0	0
GS | Product revenue
Disaggregation of Revenue [Line Items]
Revenue									$ 16,202	$ 12,053	$ 16,201