Segment Information - Information About Reportable Segments (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)	Sep. 30, 2024 USD ($) segment	Sep. 30, 2023 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment			3
Operating income	$ 245,824,000	$ 244,417,000	$ 838,492,000	$ 935,481,000
Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenues			0
Operating Segments
Segment Reporting Information [Line Items]
Revenues	1,484,306,000	1,408,784,000	4,552,292,000	4,320,838,000
Operating income	1,015,432,000	963,149,000	3,132,495,000	2,971,312,000
Operating Segments | Research
Segment Reporting Information [Line Items]
Revenues	1,280,908,000	1,218,739,000	3,815,072,000	3,643,815,000
Operating income	943,426,000	894,149,000	2,821,116,000	2,678,945,000
Operating Segments | Conferences
Segment Reporting Information [Line Items]
Revenues	75,776,000	57,200,000	331,929,000	290,739,000
Operating income	30,489,000	20,449,000	161,856,000	145,687,000
Operating Segments | Consulting
Segment Reporting Information [Line Items]
Revenues	127,622,000	132,845,000	405,291,000	386,284,000
Operating income	41,517,000	48,551,000	149,523,000	146,680,000
Corporate and Reconciling Items
Segment Reporting Information [Line Items]
Operating income	$ (769,608,000)	$ (718,732,000)	$ (2,294,003,000)	$ (2,035,831,000)