Reportable Segments - Operations of Segments (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Total Net service revenues	$ 342,586	$ 341,053	$ 340,309	$ 333,992	$ 325,998	$ 336,460	$ 335,047	$ 322,201	$ 1,357,940	$ 1,319,706	$ 1,319,238
Gross profit	$ 102,627	$ 104,375	$ 103,919	$ 97,135	$ 99,648	$ 105,380	$ 106,282	$ 97,189	408,056	408,499	414,114
Operating expenses									343,962	352,544	342,442
Income before income taxes									64,094	55,955	71,672
Flex revenues
Segment Reporting Information [Line Items]
Total Net service revenues									1,310,263	1,269,307	1,265,167
Direct Hire revenues
Segment Reporting Information [Line Items]
Total Net service revenues									47,677	50,399	54,071
Operating Segments | Tech
Segment Reporting Information [Line Items]
Total Net service revenues									907,511	883,477	895,942
Gross profit									257,118	255,842	261,721
Operating Segments | Tech | Flex revenues
Segment Reporting Information [Line Items]
Total Net service revenues									887,675	863,434	873,609
Operating Segments | Tech | Direct Hire revenues
Segment Reporting Information [Line Items]
Total Net service revenues									19,836	20,043	22,333
Operating Segments | FA
Segment Reporting Information [Line Items]
Total Net service revenues									346,135	337,601	325,924
Gross profit									118,479	120,551	119,036
Operating Segments | FA | Flex revenues
Segment Reporting Information [Line Items]
Total Net service revenues									318,294	307,245	294,186
Operating Segments | FA | Direct Hire revenues
Segment Reporting Information [Line Items]
Total Net service revenues									27,841	30,356	31,738
Operating Segments | GS
Segment Reporting Information [Line Items]
Total Net service revenues									104,294	98,628	97,372
Gross profit									32,459	32,106	33,357
Operating Segments | GS | Flex revenues
Segment Reporting Information [Line Items]
Total Net service revenues									104,294	98,628	97,372
Operating Segments | GS | Direct Hire revenues
Segment Reporting Information [Line Items]
Total Net service revenues									$ 0	$ 0	$ 0