Business Segments - Reconciliation of Revenue and Operating Income by Reportable Segment (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Segment Reporting Information [Line Items]
Service revenues	$ 1,506,691	$ 1,468,530
Operating income	131,144	146,229
Amortization of intangible assets	338	342
Interest income, net	(957)	(1,496)
Income before income taxes	131,763	147,383
Operating Segments \| Temporary and consultant staffing
Segment Reporting Information [Line Items]
Service revenues	1,092,120	1,084,627
Operating income	93,764	106,018
Operating Segments \| Permanent placement staffing
Segment Reporting Information [Line Items]
Service revenues	120,489	131,562
Operating income	10,911	21,557
Operating Segments \| Risk consulting and internal audit services
Segment Reporting Information [Line Items]
Service revenues	294,082	252,341
Operating income	$ 26,469	$ 18,654

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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