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Segment Information - Information About Reportable Segments (Details) | 3 Months Ended
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Mar. 31, 2021 USD ($) segment | Mar. 31, 2020 USD ($)
Segment Reporting Information [Line Items] |  |
Number of operating segments | segment | 3 |
Operating income | $ 225,412,000 | $ 124,718,000
Intersegment revenues |  |
Segment Reporting Information [Line Items] |  |
Revenues | 0 | 0
Reportable segments |  |
Segment Reporting Information [Line Items] |  |
Revenues | 1,104,038,000 | 1,018,891,000
Operating income | 777,366,000 | 676,791,000
Corporate and other expenses |  |
Segment Reporting Information [Line Items] |  |
Operating income | (551,954,000) | (552,073,000)
Research | Reportable segments |  |
Segment Reporting Information [Line Items] |  |
Revenues | 979,732,000 | 909,291,000
Operating income | 724,372,000 | 653,469,000
Conferences | Reportable segments |  |
Segment Reporting Information [Line Items] |  |
Revenues | 24,802,000 | 13,870,000
Operating income | 13,896,000 | (6,060,000)
Consulting | Reportable segments |  |
Segment Reporting Information [Line Items] |  |
Revenues | 99,504,000 | 95,730,000
Operating income | $ 39,098,000 | $ 29,382,000

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- Definition Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 18 -URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8672-108599 + Details
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