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Business Segments Disaggregated Revenue (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2019 | Sep. 30, 2019 | Jun. 30, 2019 | Mar. 31, 2019 | Dec. 31, 2018 | Sep. 30, 2018 | Jun. 30, 2018 | [1] | Mar. 31, 2018 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Revenue from External Customer [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenues | $ 1,025.2 | $ 1,002.7 | $ 972.3 | $ 923.7 | $ 929.7 | $ 906.4 | $ 878.5 | $ 685.2 | $ 3,923.9 | $ 3,399.8 | $ 2,626.0
Apex |  |  |  |  |  |  |  |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 2,520.0 | 2,300.3 | 2,037.2
Apex | Assignment [Member] |  |  |  |  |  |  |  |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 2,463.6 | 2,244.5 | 1,993.3
Apex | PermanentPlacement [Member] |  |  |  |  |  |  |  |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 56.4 | 55.8 | 43.9
Oxford |  |  |  |  |  |  |  |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 605.7 | 606.5 | 588.8
Oxford | Assignment [Member] |  |  |  |  |  |  |  |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 522.7 | 516.0 | 503.1
Oxford | PermanentPlacement [Member] |  |  |  |  |  |  |  |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 83.0 | 90.5 | 85.7
ECS |  |  |  |  |  |  |  |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 798.2 | 493.0 | 0.0
ECS | Fixed-price Contract [Member] |  |  |  |  |  |  |  |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 214.0 | 133.1 | 0.0
ECS | Time-and-materials Contract [Member] |  |  |  |  |  |  |  |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 267.8 | 143.4 | 0.0
ECS | Cost-plus-fixed-fee Contract [Member] |  |  |  |  |  |  |  |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | $ 316.4 | $ 216.5 | $ 0.0
[1] | ECS was acquired on April 2, 2018, see Note 4 – Acquisitions.

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