Business Segments - Reconciliation of Revenue and Operating Income by Reportable Segment (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Segment Reporting Information [Line Items]
Net service revenues	$ 1,468,530	$ 1,395,333
Operating income	146,229	134,367
Amortization of intangible assets	342	463
Interest income, net	(1,496)	(735)
Income before income taxes	147,383	134,639
Operating Segments \| Temporary and consultant staffing
Segment Reporting Information [Line Items]
Net service revenues	1,084,627	1,066,288
Operating income	106,018	96,723
Operating Segments \| Permanent placement staffing
Segment Reporting Information [Line Items]
Net service revenues	131,562	121,400
Operating income	21,557	22,379
Operating Segments \| Risk consulting and internal audit services
Segment Reporting Information [Line Items]
Net service revenues	252,341	207,645
Operating income	$ 18,654	$ 15,265

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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