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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Service revenues	$ 1,537,385	$ 1,552,132	$ 1,516,385	$ 1,468,530	$ 1,481,658	$ 1,466,226	$ 1,457,054	$ 1,395,333	$ 6,074,432	$ 5,800,271	$ 5,266,789
Operating income									621,751	588,925	517,280
Amortization of intangible assets									1,361	1,705	1,563
Interest income, net									(5,125)	(4,382)	(1,799)
Income before income taxes	$ 154,247	$ 163,782	$ 160,103	$ 147,383	$ 154,983	$ 151,905	$ 150,075	$ 134,639	625,515	591,602	517,516
Temporary and consultant staffing
Segment Reporting Information [Line Items]
Service revenues									4,412,125	4,330,566	4,011,042
Operating income									410,153	404,800	355,700
Permanent placement staffing
Segment Reporting Information [Line Items]
Service revenues									533,432	511,989	439,214
Operating income									83,885	90,801	77,673
Risk consulting and internal audit services
Segment Reporting Information [Line Items]
Service revenues									1,128,875	957,716	816,533
Operating income									$ 127,713	$ 93,324	$ 83,907