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Goodwill and Intangible Assets - Changes to the Carrying Amount of Goodwill by Segment (Details) - USD ($)
 3 Months Ended
Mar. 31, 2024
Sep. 30, 2023
Goodwill [Line Items]    
Goodwill impairment   $ 0
Goodwill [Roll Forward]    
Beginning balance $ 2,937,260,000  
Foreign currency translation impact (2,862,000)  
Ending balance 2,934,398,000  
Goodwill, accumulated impairment losses 0  
Research    
Goodwill [Roll Forward]    
Beginning balance 2,657,549,000  
Foreign currency translation impact (2,634,000)  
Ending balance 2,654,915,000  
Conferences    
Goodwill [Roll Forward]    
Beginning balance 183,997,000  
Foreign currency translation impact (34,000)  
Ending balance 183,963,000  
Consulting    
Goodwill [Roll Forward]    
Beginning balance 95,714,000  
Foreign currency translation impact (194,000)  
Ending balance $ 95,520,000