Business Segments - Reconciliation of Revenue and Operating Income by Reportable Segment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information [Line Items]
Service revenues	$ 1,712,566	$ 1,189,897	$ 4,691,527	$ 3,804,914
Segment income	227,992	102,642	583,217	292,035
Amortization of intangible assets	572	334	1,724	1,002
Interest income, net	(238)	(202)	(145)	(1,264)
Income before income taxes	227,658	102,510	581,638	292,297
Temporary and consultant staffing
Segment Reporting Information [Line Items]
Service revenues	1,054,701	781,391	2,922,257	2,626,897
Segment income	110,010	43,779	279,697	165,933
Permanent placement staffing
Segment Reporting Information [Line Items]
Service revenues	156,444	87,203	411,788	278,722
Segment income	31,030	10,128	79,264	20,791
Risk consulting and internal audit services
Segment Reporting Information [Line Items]
Service revenues	501,421	321,303	1,357,482	899,295
Segment income	$ 86,952	$ 48,735	$ 224,256	$ 105,311