Goodwill and Intangible Assets - Changes to Carrying Amount of Goodwill by Reporting Unit (Details) - USD ($)	3 Months Ended
	Mar. 31, 2022	Sep. 30, 2021
Goodwill [Line Items]
Goodwill impairment		$ 0
Goodwill [Roll Forward]
Beginning balance	$ 2,951,317,000
Foreign currency translation impact	(1,996,000)
Ending balance	2,949,321,000
Goodwill, accumulated impairment losses	0
Research
Goodwill [Roll Forward]
Beginning balance	2,670,934,000
Foreign currency translation impact	(1,652,000)
Ending balance	2,669,282,000
Conferences
Goodwill [Roll Forward]
Beginning balance	184,021,000
Foreign currency translation impact	(27,000)
Ending balance	183,994,000
Consulting
Goodwill [Roll Forward]
Beginning balance	96,362,000
Foreign currency translation impact	(317,000)
Ending balance	$ 96,045,000