Segment Information - Information About Reportable Segments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Operating income (loss)	$ 259,715	$ (6,329)	$ 305,141
Operating Segments
Segment Reporting Information [Line Items]
Revenues	3,975,454	3,311,494	2,444,540
Total segment gross contribution	2,518,973	2,000,386	1,512,000
Operating Segments | Research
Segment Reporting Information [Line Items]
Revenues	3,105,764	2,471,280	1,857,001
Total segment gross contribution	2,144,097	1,653,014	1,285,611
Operating Segments | Conferences
Segment Reporting Information [Line Items]
Revenues	410,461	337,903	268,605
Total segment gross contribution	207,260	163,480	136,655
Operating Segments | Consulting
Segment Reporting Information [Line Items]
Revenues	353,667	327,661	318,934
Total segment gross contribution	102,541	93,643	89,734
Operating Segments | Other
Segment Reporting Information [Line Items]
Revenues	105,562	174,650	0
Total segment gross contribution	65,075	90,249	0
Corporate and Reconciling Items
Segment Reporting Information [Line Items]
Corporate and other expenses	$ (2,259,258)	$ (2,006,715)	$ (1,206,859)