Business Segments - Reconciliation of Revenue and Operating Income by Reportable Segment (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Net service revenues	$ 1,552,132	$ 1,466,226	$ 4,537,047	$ 4,318,613
Operating income	162,891	151,138	468,522	435,016
Amortization of intangible assets	339	429	1,022	1,334
Interest income, net	(1,230)	(1,196)	(3,768)	(2,937)
Income before income taxes	163,782	151,905	471,268	436,619
Operating Segments \| Temporary and consultant staffing
Segment Reporting Information [Line Items]
Net service revenues	1,118,461	1,083,766	3,305,800	3,238,028
Operating income	101,428	102,707	312,684	303,000
Operating Segments \| Permanent placement staffing
Segment Reporting Information [Line Items]
Net service revenues	134,582	129,667	407,038	386,105
Operating income	21,817	22,895	68,718	72,710
Operating Segments \| Risk consulting and internal audit services
Segment Reporting Information [Line Items]
Net service revenues	299,089	252,793	824,209	694,480
Operating income	$ 39,646	$ 25,536	$ 87,120	$ 59,306

The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

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