Business Segments - Reconciliation of Revenue and Operating Income by Reportable Segment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Service revenues	$ 1,189,897	$ 1,552,132	$ 3,804,914	$ 4,537,047
Segment income	102,642	162,891	292,035	468,522
Amortization of intangible assets	334	339	1,002	1,022
Interest income, net	(202)	(1,230)	(1,264)	(3,768)
Income before income taxes	102,510	163,782	292,297	471,268
Operating Segments | Temporary and consultant staffing
Segment Reporting Information [Line Items]
Service revenues	781,391	1,118,461	2,626,897	3,305,800
Segment income	43,779	101,428	165,933	312,684
Operating Segments | Permanent placement staffing
Segment Reporting Information [Line Items]
Service revenues	87,203	134,582	278,722	407,038
Segment income	10,128	21,817	20,791	68,718
Operating Segments | Risk consulting and internal audit services
Segment Reporting Information [Line Items]
Service revenues	321,303	299,089	899,295	824,209
Segment income	$ 48,735	$ 39,646	$ 105,311	$ 87,120