Revenue and Related Matters - Timing Of Revenue Recognition Liabilities per Segment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Total revenues	$ 994,618	$ 1,000,502	$ 2,986,644	$ 3,041,828
Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	897,647	847,288	2,687,461	2,512,424
Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	96,971	153,214	299,183	529,404
Research
Disaggregation of Revenue [Line Items]
Total revenues	892,719	840,998	2,677,339	2,492,427
Research | Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	823,658	769,718	2,463,434	2,276,783
Research | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	69,061	71,280	213,905	215,644
Conferences
Disaggregation of Revenue [Line Items]
Total revenues	12,738	66,286	26,925	259,392
Conferences | Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0	0
Conferences | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	12,738	66,286	26,925	259,392
Consulting
Disaggregation of Revenue [Line Items]
Total revenues	89,161	93,218	282,380	290,009
Consulting | Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	73,989	77,570	224,027	235,641
Consulting | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	$ 15,172	$ 15,648	$ 58,353	$ 54,368