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Goodwill and Intangible Assets - Changes to Carrying Amount of Goodwill by Reporting Unit (Details) | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2021 USD ($) | Sep. 30, 2021 USD ($)
Goodwill [Line Items] |  |
Goodwill impairment | $ 0 |
Goodwill [Roll Forward] |  |
Beginning balance |  | $ 2,945,547,000
Additions due to an acquisition |  | 11,045,000
Foreign currency translation impact |  | (3,665,000)
Ending balance | 2,952,927,000 | 2,952,927,000
Goodwill, accumulated impairment losses | 0 | 0
Research |  |
Goodwill [Roll Forward] |  |
Beginning balance |  | 2,664,732,000
Additions due to an acquisition |  | 11,045,000
Foreign currency translation impact |  | (3,510,000)
Ending balance | 2,672,267,000 | 2,672,267,000
Conferences |  |
Goodwill [Roll Forward] |  |
Beginning balance |  | 184,091,000
Additions due to an acquisition |  | 0
Foreign currency translation impact |  | (44,000)
Ending balance | 184,047,000 | 184,047,000
Consulting |  |
Goodwill [Roll Forward] |  |
Beginning balance |  | 96,724,000
Additions due to an acquisition |  | 0
Foreign currency translation impact |  | (111,000)
Ending balance | $ 96,613,000 | $ 96,613,000

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- Definition Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(10)(1)) -URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=120320667&loc=SL49117168-202975 Reference 3: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 852 -SubTopic 10 -Section 55 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=84165509&loc=d3e56426-112766 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=99380562&loc=d3e13770-109266 + Details
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