Segment Information - Information About Reportable Segments (Details)	3 Months Ended	6 Months Ended
Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($) segment	Jun. 30, 2020 USD ($)
Segment Reporting Information [Line Items]
Number of operating segments \| segment			3
Operating income	$ 274,714,000	$ 99,651,000	$ 500,126,000	$ 224,370,000
Intersegment revenues
Segment Reporting Information [Line Items]
Revenues			0
Reportable segments
Segment Reporting Information [Line Items]
Revenues	1,167,319,000	973,135,000	2,271,357,000	1,992,026,000
Operating income	827,503,000	654,936,000	1,604,869,000	1,331,729,000
Corporate and other expenses
Segment Reporting Information [Line Items]
Operating income	(552,789,000)	(555,285,000)	(1,104,743,000)	(1,107,359,000)
Research \| Reportable segments
Segment Reporting Information [Line Items]
Revenues	1,003,238,000	875,329,000	1,982,970,000	1,784,620,000
Operating income	742,131,000	632,624,000	1,466,503,000	1,286,094,000
Conferences \| Reportable segments
Segment Reporting Information [Line Items]
Revenues	58,179,000	317,000	82,981,000	14,187,000
Operating income	42,602,000	(11,230,000)	56,498,000	(17,290,000)
Consulting \| Reportable segments
Segment Reporting Information [Line Items]
Revenues	105,902,000	97,489,000	205,406,000	193,219,000
Operating income	$ 42,770,000	$ 33,542,000	$ 81,868,000	$ 62,925,000