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Business Segments Disaggregated Revenue (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenue from External Customer [Line Items]
Revenues	$ 3,950.6	$ 3,923.9	$ 3,399.8
Apex
Revenue from External Customer [Line Items]
Revenues	2,421.2	2,520.0	2,300.3
Apex | Assignment
Revenue from External Customer [Line Items]
Revenues	2,040.3	2,190.9	2,050.1
Apex | Consulting
Revenue from External Customer [Line Items]
Revenues	380.9	329.1	250.2
Oxford
Revenue from External Customer [Line Items]
Revenues	525.2	605.7	606.5
Oxford | Assignment
Revenue from External Customer [Line Items]
Revenues	468.6	549.4	558.1
Oxford | Consulting
Revenue from External Customer [Line Items]
Revenues	56.6	56.3	48.4
ECS
Revenue from External Customer [Line Items]
Revenues	1,004.2	798.2	493.0
ECS | Firm-fixed-price
Revenue from External Customer [Line Items]
Revenues	272.0	214.0	133.1
ECS | Time and materials
Revenue from External Customer [Line Items]
Revenues	322.6	267.8	143.4
ECS | Cost reimbursable
Revenue from External Customer [Line Items]
Revenues	$ 409.6	$ 316.4	$ 216.5

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 22
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8736-108599

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 32
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8933-108599

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 22
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8736-108599

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(1))
-URI http://asc.fasb.org/extlink&oid=116634182&loc=SL114868664-224227

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