Revenue and Related Matters - Timing Of Revenue Recognition Liabilities per Segment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Total revenues	$ 4,099,403	$ 4,245,321	$ 3,975,454
Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	3,609,657	3,399,978	3,232,240
Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	489,746	845,343	743,214
Research
Disaggregation of Revenue [Line Items]
Total revenues	3,602,892	3,374,548	3,105,764
Research | Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	3,313,111	3,083,936	2,851,176
Research | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	289,781	290,612	254,588
Conferences
Disaggregation of Revenue [Line Items]
Total revenues	120,140	476,869	410,461
Conferences | Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0
Conferences | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	120,140	476,869	410,461
Consulting
Disaggregation of Revenue [Line Items]
Total revenues	376,371	393,904	353,667
Consulting | Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	296,546	316,042	294,397
Consulting | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	$ 79,825	$ 77,862	59,270
Other
Disaggregation of Revenue [Line Items]
Total revenues			105,562
Other | Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues			86,667
Other | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues			$ 18,895