Business Segments - Reconciliation of Revenue and Operating Income by Reportable Segment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Reporting Information [Line Items]
Service revenues	$ 1,580,581	$ 1,108,326	$ 2,978,961	$ 2,615,017
Segment income	204,589	58,249	355,225	189,393
Amortization of intangible assets	576	330	1,152	668
Interest expense (income), net	151	(105)	105	(1,062)
Income before income taxes	203,862	58,024	353,968	189,787
Temporary and consultant staffing
Segment Reporting Information [Line Items]
Service revenues	978,281	753,386	1,867,557	1,845,506
Segment income	94,010	28,390	169,688	122,154
Permanent placement staffing
Segment Reporting Information [Line Items]
Service revenues	143,640	71,030	255,344	191,519
Segment income	30,599	(248)	48,234	10,663
Risk consulting and internal audit services
Segment Reporting Information [Line Items]
Service revenues	458,660	283,910	856,060	577,992
Segment income	$ 79,980	$ 30,107	$ 137,303	$ 56,576