Goodwill and Intangible Assets - Changes to Carrying Amount of Goodwill by Reporting Unit (Details) - USD ($)	3 Months Ended	6 Months Ended
	Sep. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Goodwill [Line Items]
Goodwill impairment	$ 0
Goodwill [Roll Forward]
Beginning balance		$ 2,937,726,000
Foreign currency translation impact		(2,117,000)
Ending balance		2,935,609,000
Goodwill, accumulated impairment losses			$ 0
Research
Goodwill [Roll Forward]
Beginning balance		2,651,060,000
Foreign currency translation impact		(1,586,000)
Ending balance		2,649,474,000
Conferences
Goodwill [Roll Forward]
Beginning balance		189,641,000
Foreign currency translation impact		(3,000)
Ending balance		189,638,000
Consulting
Goodwill [Roll Forward]
Beginning balance		97,025,000
Foreign currency translation impact		(528,000)
Ending balance		$ 96,497,000