Goodwill and Intangible Assets - Changes to Carrying Amount of Goodwill by Reporting Unit (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Sep. 30, 2019	Mar. 31, 2019
Goodwill [Line Items]
Goodwill impairment		$ 0
Goodwill [Roll Forward]
Beginning balance	$ 2,937,726,000
Foreign currency translation impact	(10,060,000)
Ending balance	2,927,666,000
Goodwill, accumulated impairment losses			$ 0
Research
Goodwill [Roll Forward]
Beginning balance	2,651,060,000
Foreign currency translation impact	(8,718,000)
Ending balance	2,642,342,000
Conferences
Goodwill [Roll Forward]
Beginning balance	189,641,000
Foreign currency translation impact	(1,278,000)
Ending balance	188,363,000
Consulting
Goodwill [Roll Forward]
Beginning balance	97,025,000
Foreign currency translation impact	(64,000)
Ending balance	$ 96,961,000