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Goodwill and Intangible Assets - Changes to the Carrying Amount of Goodwill by Segment (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019
Goodwill [Roll Forward] |  |
Beginning Balance | $ 2,937,726 | $ 2,923,136
Additions due to acquisition |  | 19,293
Foreign currency translation impact and other | 7,821 | (4,703)
Ending Balance | 2,945,547 | 2,937,726
Research |  |
Goodwill [Roll Forward] |  |
Beginning Balance | 2,651,060 | 2,638,418
Additions due to acquisition |  | 17,557
Foreign currency translation impact and other | 13,672 | (4,915)
Ending Balance | 2,664,732 | 2,651,060
Conferences |  |
Goodwill [Roll Forward] |  |
Beginning Balance | 189,641 | 187,654
Additions due to acquisition |  | 1,736
Foreign currency translation impact and other | (5,550) | 251
Ending Balance | 184,091 | 189,641
Consulting |  |
Goodwill [Roll Forward] |  |
Beginning Balance | 97,025 | 97,064
Additions due to acquisition |  | 0
Foreign currency translation impact and other | (301) | (39)
Ending Balance | $ 96,724 | $ 97,025

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