Goodwill and Intangible Assets - Changes to Carrying Amount of Goodwill by Reporting Unit (Details) - USD ($)	3 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2020
Goodwill [Line Items]
Goodwill impairment	$ 0
Goodwill [Roll Forward]
Beginning balance		$ 2,937,726,000
Foreign currency translation impact		982,000
Ending balance		2,938,708,000
Goodwill, accumulated impairment losses	$ 0
Research
Goodwill [Roll Forward]
Beginning balance		2,651,060,000
Foreign currency translation impact		1,187,000
Ending balance		2,652,247,000
Conferences
Goodwill [Roll Forward]
Beginning balance		189,641,000
Foreign currency translation impact		29,000
Ending balance		189,670,000
Consulting
Goodwill [Roll Forward]
Beginning balance		97,025,000
Foreign currency translation impact		(234,000)
Ending balance		$ 96,791,000