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Acquisitions and Divestitures - Divestitures (Details) $ in Thousands |  |  |  | 12 Months Ended
--- | --- | --- | --- | ---
Aug. 31, 2018 USD ($) | May 01, 2018 USD ($) | Apr. 03, 2018 USD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) business
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Gain on disposal |  |  |  | $ 0 | $ (2,075) | $ 45,447
Disposed of by Sale |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Number of businesses sold | business |  |  |  |  |  | 3
Divestiture, revenue contributed |  |  |  |  |  | $ 97,300
Divestiture, gross contribution |  |  |  |  |  | 60,500
Disposed of by Sale | CEB Challenger |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Sale of business, consideration | $ 119,100 |  |  |  |  |
Proceeds from divestiture of businesses | 116,000 |  |  |  |  |
Gain on disposal | $ 8,300 |  |  |  |  |
Disposed of by Sale | CEB Workforce Survey and Analytics |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Sale of business, consideration |  | $ 28,000 |  |  |  |
Proceeds from divestiture of businesses |  | 26,400 |  |  |  |
Gain on disposal |  | $ 8,800 |  |  |  |
Disposed of by Sale | CEB Talent Assessment |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Sale of business, consideration |  |  | $ 403,000 |  |  |
Proceeds from divestiture of businesses |  |  | 375,800 |  |  |
Gain on disposal |  |  | $ 15,500 |  |  |
Disposed of by Sale | Other Certain Assets In CEB Transaction |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Proceeds from sale of assets |  |  |  |  |  | 8,600
Gain on sale of assets |  |  |  |  |  | $ 12,800

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