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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Net service revenues	$ 1,481,658	$ 1,466,226	$ 1,457,054	$ 1,395,333	$ 1,346,282	$ 1,324,709	$ 1,308,428	$ 1,287,370	$ 5,800,271	$ 5,266,789	$ 5,250,399
Operating income									588,925	517,280	554,459
Amortization of intangible assets									1,705	1,563	1,237
Interest income, net									(4,382)	(1,799)	(888)
Income before income taxes	$ 154,983	$ 151,905	$ 150,075	$ 134,639	$ 129,038	$ 132,270	$ 130,707	$ 125,501	591,602	517,516	554,110
Temporary and consultant staffing
Segment Reporting Information [Line Items]
Net service revenues									4,330,566	4,011,042	4,026,777
Operating income									404,800	355,700	393,704
Permanent placement staffing
Segment Reporting Information [Line Items]
Net service revenues									511,989	439,214	419,314
Operating income									90,801	77,673	80,001
Risk consulting and internal audit services
Segment Reporting Information [Line Items]
Net service revenues									957,716	816,533	804,308
Operating income									$ 93,324	$ 83,907	$ 80,754