Revenue and Related Matters - Timing Of Revenue Recognition Liabilities per Segment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Total revenues	$ 4,245,321	$ 3,975,454	$ 3,311,494
Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	3,399,978	3,232,240	2,686,428
Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	845,343	743,214	625,066
Research
Disaggregation of Revenue [Line Items]
Total revenues	3,374,548	3,105,764	2,471,280
Research | Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	3,083,936	2,851,176	2,275,377
Research | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	290,612	254,588	195,903
Conferences
Disaggregation of Revenue [Line Items]
Total revenues	476,869	410,461	337,903
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	476,869	410,461	337,903
Consulting
Disaggregation of Revenue [Line Items]
Total revenues	393,904	353,667	327,661
Consulting | Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	316,042	294,397	269,720
Consulting | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	$ 77,862	59,270	57,941
Other
Disaggregation of Revenue [Line Items]
Total revenues		105,562	174,650
Other | Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues		86,667	141,331
Other | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues		$ 18,895	$ 33,319