Segment Information - Information About Reportable Segments (Details)	3 Months Ended
	Mar. 31, 2022 USD ($) segment	Mar. 31, 2021 USD ($)
Segment Reporting Information [Line Items]
Number of operating segments | segment	3
Operating income	$ 217,247,000	$ 225,412,000
Intersegment revenues
Segment Reporting Information [Line Items]
Revenues	0
Reportable segments
Segment Reporting Information [Line Items]
Revenues	1,262,740,000	1,104,038,000
Operating income	897,515,000	777,366,000
Corporate and other expenses
Segment Reporting Information [Line Items]
Operating income	(680,268,000)	(551,954,000)
Research | Reportable segments
Segment Reporting Information [Line Items]
Revenues	1,136,380,000	979,732,000
Operating income	849,379,000	724,372,000
Conferences | Reportable segments
Segment Reporting Information [Line Items]
Revenues	10,354,000	24,802,000
Operating income	(2,876,000)	13,896,000
Consulting | Reportable segments
Segment Reporting Information [Line Items]
Revenues	116,006,000	99,504,000
Operating income	$ 51,012,000	$ 39,098,000