Segment Information - Information About Reportable Segments (Details)	3 Months Ended
	Mar. 31, 2021 USD ($) segment	Mar. 31, 2020 USD ($)
Segment Reporting Information [Line Items]
Number of operating segments | segment	3
Operating income	$ 225,412,000	$ 124,718,000
Intersegment revenues
Segment Reporting Information [Line Items]
Revenues	0	0
Reportable segments
Segment Reporting Information [Line Items]
Revenues	1,104,038,000	1,018,891,000
Operating income	777,366,000	676,791,000
Corporate and other expenses
Segment Reporting Information [Line Items]
Operating income	(551,954,000)	(552,073,000)
Research | Reportable segments
Segment Reporting Information [Line Items]
Revenues	979,732,000	909,291,000
Operating income	724,372,000	653,469,000
Conferences | Reportable segments
Segment Reporting Information [Line Items]
Revenues	24,802,000	13,870,000
Operating income	13,896,000	(6,060,000)
Consulting | Reportable segments
Segment Reporting Information [Line Items]
Revenues	99,504,000	95,730,000
Operating income	$ 39,098,000	$ 29,382,000