Segment Information - Information About Reportable Segments (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2022 USD ($)	Jun. 30, 2021 USD ($)	Jun. 30, 2022 USD ($) segment	Jun. 30, 2021 USD ($)
Segment Reporting Information [Line Items]
Number of operating segments | segment			3
Operating income	$ 297,122,000	$ 274,714,000	$ 514,369,000	$ 500,126,000
Intersegment revenues
Segment Reporting Information [Line Items]
Revenues			0
Reportable segments
Segment Reporting Information [Line Items]
Revenues	1,376,521,000	1,167,319,000	2,639,261,000	2,271,357,000
Operating income	967,714,000	827,503,000	1,865,229,000	1,604,869,000
Corporate and other expenses
Segment Reporting Information [Line Items]
Operating income	(670,592,000)	(552,789,000)	(1,350,860,000)	(1,104,743,000)
Research | Reportable segments
Segment Reporting Information [Line Items]
Revenues	1,142,329,000	1,003,238,000	2,278,709,000	1,982,970,000
Operating income	843,965,000	742,131,000	1,693,344,000	1,466,503,000
Conferences | Reportable segments
Segment Reporting Information [Line Items]
Revenues	113,525,000	58,179,000	123,879,000	82,981,000
Operating income	73,526,000	42,602,000	70,650,000	56,498,000
Consulting | Reportable segments
Segment Reporting Information [Line Items]
Revenues	120,667,000	105,902,000	236,673,000	205,406,000
Operating income	$ 50,223,000	$ 42,770,000	$ 101,235,000	$ 81,868,000