Goodwill and Intangible Assets - Changes to Carrying Amount of Goodwill by Reporting Unit (Details) - USD ($)	3 Months Ended
	Mar. 31, 2021	Sep. 30, 2020
Goodwill [Line Items]
Goodwill impairment		$ 0
Goodwill [Roll Forward]
Beginning balance	$ 2,945,547,000
Foreign currency translation impact	(2,047,000)
Ending balance	2,943,500,000
Goodwill, accumulated impairment losses	0
Research
Goodwill [Roll Forward]
Beginning balance	2,664,732,000
Foreign currency translation impact	(1,917,000)
Ending balance	2,662,815,000
Conferences
Goodwill [Roll Forward]
Beginning balance	184,091,000
Foreign currency translation impact	(38,000)
Ending balance	184,053,000
Consulting
Goodwill [Roll Forward]
Beginning balance	96,724,000
Foreign currency translation impact	(92,000)
Ending balance	$ 96,632,000