Segment Information - Information About Reportable Segments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Operating income	$ 915,751	$ 490,150	$ 370,087
Operating Segments
Segment Reporting Information [Line Items]
Revenues	4,733,962	4,099,403	4,245,321
Operating income	3,329,516	2,770,898	2,711,927
Operating Segments | Research
Segment Reporting Information [Line Items]
Revenues	4,101,392	3,602,892	3,374,548
Operating income	3,036,925	2,597,852	2,351,720
Operating Segments | Conferences
Segment Reporting Information [Line Items]
Revenues	214,449	120,140	476,869
Operating income	133,748	57,302	241,757
Operating Segments | Consulting
Segment Reporting Information [Line Items]
Revenues	418,121	376,371	393,904
Operating income	158,843	115,744	118,450
Corporate and Reconciling Items
Segment Reporting Information [Line Items]
Operating income	$ (2,413,765)	$ (2,280,748)	$ (2,341,840)