Business Segments - Reconciliation of Revenue and Operating Income by Reportable Segment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Segment Reporting Information [Line Items]
Service revenues	$ 1,398,380	$ 1,506,691
Segment income	150,637	131,144
Amortization of intangible assets	576	338
Interest income, net	(45)	(957)
Income before income taxes	150,106	131,763
Temporary and consultant staffing
Segment Reporting Information [Line Items]
Service revenues	889,275	1,092,120
Segment income	75,535	93,764
Permanent placement staffing
Segment Reporting Information [Line Items]
Service revenues	111,703	120,489
Segment income	17,780	10,911
Risk consulting and internal audit services
Segment Reporting Information [Line Items]
Service revenues	397,402	294,082
Segment income	$ 57,322	$ 26,469