Business Segments - Reconciliation of Revenue and Operating Income by Reportable Segment to Consolidated Results (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Service revenues	$ 6,461,444	$ 5,109,000	$ 6,074,432
Operating income	805,824	421,758	621,751
Amortization of intangible assets	2,241	1,219	1,361
Interest income, net	(197)	(1,343)	(5,125)
Income before income taxes	803,780	421,882	625,515
Temporary and consultant staffing
Segment Reporting Information [Line Items]
Service revenues	4,038,743	3,477,335	4,412,125
Operating income	393,872	237,279	410,153
Permanent placement staffing
Segment Reporting Information [Line Items]
Service revenues	569,921	370,109	533,432
Operating income	106,465	28,799	83,885
Risk consulting and internal audit services
Segment Reporting Information [Line Items]
Service revenues	1,852,780	1,261,556	1,128,875
Operating income	$ 305,487	$ 155,680	$ 127,713