Segment Information - Information About Reportable Segments (Details)	3 Months Ended
	Mar. 31, 2020 USD ($) segment	Mar. 31, 2019 USD ($)
Segment Reporting Information [Line Items]
Number of operating segments | segment	3
Operating income	$ 124,718,000	$ 48,799,000
Intersegment revenues
Segment Reporting Information [Line Items]
Revenues	0
Reportable segments
Segment Reporting Information [Line Items]
Revenues	1,018,891,000	970,444,000
Operating income	676,791,000	622,762,000
Corporate and other expenses
Segment Reporting Information [Line Items]
Operating income	(552,073,000)	(573,963,000)
Research | Reportable segments
Segment Reporting Information [Line Items]
Revenues	909,291,000	825,374,000
Operating income	653,469,000	575,168,000
Conferences | Reportable segments
Segment Reporting Information [Line Items]
Revenues	13,870,000	51,932,000
Operating income	(6,060,000)	18,876,000
Consulting | Reportable segments
Segment Reporting Information [Line Items]
Revenues	95,730,000	93,138,000
Operating income	$ 29,382,000	$ 28,718,000