Segment Information - Information About Reportable Segments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Operating income (loss)	$ 370,087	$ 259,715	$ (6,329)
Operating Segments
Segment Reporting Information [Line Items]
Revenues	4,245,321	3,975,454	3,311,494
Operating income (loss)	2,711,927	2,518,973	2,000,386
Operating Segments | Research
Segment Reporting Information [Line Items]
Revenues	3,374,548	3,105,764	2,471,280
Operating income (loss)	2,351,720	2,144,097	1,653,014
Operating Segments | Conferences
Segment Reporting Information [Line Items]
Revenues	476,869	410,461	337,903
Operating income (loss)	241,757	207,260	163,480
Operating Segments | Consulting
Segment Reporting Information [Line Items]
Revenues	393,904	353,667	327,661
Operating income (loss)	118,450	102,541	93,643
Operating Segments | Other
Segment Reporting Information [Line Items]
Revenues		105,562	174,650
Operating income (loss)		65,075	90,249
Corporate and Reconciling Items
Segment Reporting Information [Line Items]
Operating income (loss)	$ (2,341,840)	$ (2,259,258)	$ (2,006,715)