Business Segments - Reconciliation of Revenue and Operating Income by Reportable Segment to Consolidated Results (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Service revenues	$ 1,304,086	$ 1,189,897	$ 1,108,326	$ 1,506,691	$ 1,537,385	$ 1,552,132	$ 1,516,385	$ 1,468,530	$ 5,109,000	$ 6,074,432	$ 5,800,271
Segment income									421,758	621,751	588,925
Amortization of intangible assets									1,219	1,361	1,705
Interest income, net									(1,343)	(5,125)	(4,382)
Income before income taxes	$ 129,585	$ 102,510	$ 58,024	$ 131,763	$ 154,247	$ 163,782	$ 160,103	$ 147,383	421,882	625,515	591,602
Temporary and consultant staffing
Segment Reporting Information [Line Items]
Service revenues									3,477,335	4,412,125	4,330,566
Segment income									237,279	410,153	404,800
Permanent placement staffing
Segment Reporting Information [Line Items]
Service revenues									370,109	533,432	511,989
Segment income									28,799	83,885	90,801
Risk consulting and internal audit services
Segment Reporting Information [Line Items]
Service revenues									1,261,556	1,128,875	957,716
Segment income									$ 155,680	$ 127,713	$ 93,324