Business Segments - Reconciliation of Revenue and Operating Income by Reportable Segment (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information [Line Items]
Net service revenues	$ 1,516,385	$ 1,457,054	$ 2,984,915	$ 2,852,387
Operating income	159,402	149,511	305,631	283,878
Amortization of intangible assets	341	442	683	905
Interest income, net	(1,042)	(1,006)	(2,538)	(1,741)
Income before income taxes	160,103	150,075	307,486	284,714
Operating Segments \| Temporary and consultant staffing
Segment Reporting Information [Line Items]
Net service revenues	1,102,712	1,087,974	2,187,339	2,154,262
Operating income	105,238	103,570	211,256	200,293
Operating Segments \| Permanent placement staffing
Segment Reporting Information [Line Items]
Net service revenues	140,894	135,038	272,456	256,438
Operating income	25,344	27,436	46,901	49,815
Operating Segments \| Risk consulting and internal audit services
Segment Reporting Information [Line Items]
Net service revenues	272,779	234,042	525,120	441,687
Operating income	$ 28,820	$ 18,505	$ 47,474	$ 33,770

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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