Business Segments - Reconciliation of Revenue and Operating Income by Reportable Segment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information [Line Items]
Service revenues	$ 1,108,326	$ 1,516,385	$ 2,615,017	$ 2,984,915
Operating income	58,249	159,402	189,393	305,631
Amortization of intangible assets	330	341	668	683
Interest income, net	(105)	(1,042)	(1,062)	(2,538)
Income before income taxes	58,024	160,103	189,787	307,486
Operating Segments | Temporary and consultant staffing
Segment Reporting Information [Line Items]
Service revenues	753,386	1,102,712	1,845,506	2,187,339
Operating income	28,390	105,238	122,154	211,256
Operating Segments | Permanent placement staffing
Segment Reporting Information [Line Items]
Service revenues	71,030	140,894	191,519	272,456
Operating income	(248)	25,344	10,663	46,901
Operating Segments | Risk consulting and internal audit services
Segment Reporting Information [Line Items]
Service revenues	283,910	272,779	577,992	525,120
Operating income	$ 30,107	$ 28,820	$ 56,576	$ 47,474