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