Segment Information - Information About Reportable Segments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Operating income	$ 490,150	$ 370,087	$ 259,715
Operating Segments
Segment Reporting Information [Line Items]
Revenues	4,099,403	4,245,321	3,975,454
Operating income	2,770,898	2,711,927	2,518,973
Operating Segments | Research
Segment Reporting Information [Line Items]
Revenues	3,602,892	3,374,548	3,105,764
Operating income	2,597,852	2,351,720	2,144,097
Operating Segments | Conferences
Segment Reporting Information [Line Items]
Revenues	120,140	476,869	410,461
Operating income	57,302	241,757	207,260
Operating Segments | Consulting
Segment Reporting Information [Line Items]
Revenues	376,371	393,904	353,667
Operating income	115,744	118,450	102,541
Operating Segments | Other
Segment Reporting Information [Line Items]
Revenues			105,562
Operating income			65,075
Corporate and Reconciling Items
Segment Reporting Information [Line Items]
Operating income	$ (2,280,748)	$ (2,341,840)	$ (2,259,258)