Segment Information - Information About Reportable Segments (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) segment	Sep. 30, 2020 USD ($)
Segment Reporting Information [Line Items]
Number of operating segments | segment			3
Operating income	$ 230,221,000	$ 87,650,000	$ 730,347,000	$ 312,020,000
Intersegment revenues
Segment Reporting Information [Line Items]
Revenues			0
Reportable segments
Segment Reporting Information [Line Items]
Revenues	1,156,282,000	994,618,000	3,427,639,000	2,986,644,000
Operating income	811,519,000	672,533,000	2,416,388,000	2,004,262,000
Corporate and other expenses
Segment Reporting Information [Line Items]
Operating income	(581,298,000)	(584,883,000)	(1,686,041,000)	(1,692,242,000)
Research | Reportable segments
Segment Reporting Information [Line Items]
Revenues	1,037,124,000	892,719,000	3,020,094,000	2,677,339,000
Operating income	769,091,000	642,328,000	2,235,594,000	1,928,422,000
Conferences | Reportable segments
Segment Reporting Information [Line Items]
Revenues	24,415,000	12,738,000	107,396,000	26,925,000
Operating income	11,456,000	2,044,000	67,954,000	(15,246,000)
Consulting | Reportable segments
Segment Reporting Information [Line Items]
Revenues	94,743,000	89,161,000	300,149,000	282,380,000
Operating income	$ 30,972,000	$ 28,161,000	$ 112,840,000	$ 91,086,000