Segment Information - Information About Reportable Segments (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2023 USD ($)	Jun. 30, 2022 USD ($)	Jun. 30, 2023 USD ($) segment	Jun. 30, 2022 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment			3
Operating income	$ 283,107,000	$ 297,122,000	$ 691,064,000	$ 514,369,000
Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenues			0
Operating Segments
Segment Reporting Information [Line Items]
Revenues	1,503,185,000	1,376,521,000	2,912,054,000	2,639,261,000
Operating income	1,031,053,000	967,714,000	2,008,163,000	1,865,229,000
Operating Segments | Research
Segment Reporting Information [Line Items]
Revenues	1,207,885,000	1,142,329,000	2,425,076,000	2,278,709,000
Operating income	885,282,000	843,965,000	1,784,796,000	1,693,344,000
Operating Segments | Conferences
Segment Reporting Information [Line Items]
Revenues	168,897,000	113,525,000	233,539,000	123,879,000
Operating income	98,450,000	73,526,000	125,238,000	70,650,000
Operating Segments | Consulting
Segment Reporting Information [Line Items]
Revenues	126,403,000	120,667,000	253,439,000	236,673,000
Operating income	47,321,000	50,223,000	98,129,000	101,235,000
Corporate and Reconciling Items
Segment Reporting Information [Line Items]
Operating income	$ (747,946,000)	$ (670,592,000)	$ (1,317,099,000)	$ (1,350,860,000)