Segment Information - Information About Reportable Segments (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) segment	Sep. 30, 2019 USD ($)
Segment Reporting Information [Line Items]
Number of operating segments | segment			3
Operating income	$ 87,650,000	$ 69,147,000	$ 312,020,000	$ 233,948,000
Intersegment revenues
Segment Reporting Information [Line Items]
Revenues	0	0	0	0
Reportable segments
Segment Reporting Information [Line Items]
Revenues	994,618,000	1,000,502,000	2,986,644,000	3,041,828,000
Operating income	672,533,000	636,505,000	2,004,262,000	1,946,370,000
Corporate and other expenses
Segment Reporting Information [Line Items]
Operating income	(584,883,000)	(567,358,000)	(1,692,242,000)	(1,712,422,000)
Research | Reportable segments
Segment Reporting Information [Line Items]
Revenues	892,719,000	840,998,000	2,677,339,000	2,492,427,000
Operating income	642,328,000	582,502,000	1,928,422,000	1,729,967,000
Conferences | Reportable segments
Segment Reporting Information [Line Items]
Revenues	12,738,000	66,286,000	26,925,000	259,392,000
Operating income	2,044,000	27,465,000	(15,246,000)	126,910,000
Consulting | Reportable segments
Segment Reporting Information [Line Items]
Revenues	89,161,000	93,218,000	282,380,000	290,009,000
Operating income	$ 28,161,000	$ 26,538,000	$ 91,086,000	$ 89,493,000