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Goodwill and Intangible Assets - Changes to Carrying Amount of Goodwill by Reporting Unit (Details) - USD ($)
 3 Months Ended 6 Months Ended
Sep. 30, 2020
Jun. 30, 2021
Goodwill [Line Items]    
Goodwill impairment $ 0  
Goodwill [Roll Forward]    
Beginning balance   $ 2,945,547,000
Additions due to an acquisition   11,172,000
Foreign currency translation impact   (1,253,000)
Ending balance   2,955,466,000
Goodwill, accumulated impairment losses   0
Research    
Goodwill [Roll Forward]    
Beginning balance   2,664,732,000
Additions due to an acquisition   11,172,000
Foreign currency translation impact   (1,398,000)
Ending balance   2,674,506,000
Conferences    
Goodwill [Roll Forward]    
Beginning balance   184,091,000
Additions due to an acquisition   0
Foreign currency translation impact   (20,000)
Ending balance   184,071,000
Consulting    
Goodwill [Roll Forward]    
Beginning balance   96,724,000
Additions due to an acquisition   0
Foreign currency translation impact   165,000
Ending balance   $ 96,889,000