Business Segments - Reconciliation of Revenue and Operating Income by Reportable Segment (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Segment Reporting Information [Line Items]
Net service revenues	$ 1,308,428	$ 1,344,160	$ 2,595,798	$ 2,646,785
Operating income	130,717	149,497	256,296	283,395
Amortization of intangible assets	365	314	666	602
Interest income, net	(355)	(231)	(578)	(412)
Income before income taxes	130,707	149,414	256,208	283,205
Operating Segments \| Temporary and consultant staffing
Segment Reporting Information [Line Items]
Net service revenues	998,296	1,032,083	1,985,902	2,041,248
Operating income	93,088	107,133	183,459	205,016
Operating Segments \| Permanent placement staffing
Segment Reporting Information [Line Items]
Net service revenues	112,653	113,439	216,286	219,728
Operating income	21,046	24,576	39,348	46,078
Operating Segments \| Risk consulting and internal audit services
Segment Reporting Information [Line Items]
Net service revenues	197,479	198,638	393,610	385,809
Operating income	$ 16,583	$ 17,788	$ 33,489	$ 32,301

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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6388964&loc=d3e16225-109274

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
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Amount of income (loss) from continuing operations before deduction of income tax expense (benefit) and income (loss) attributable to noncontrolling interest, and addition of income (loss) from equity method investments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
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-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

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The net amount of operating interest income (expense).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 225
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-Section S99
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