Revenue and Related Matters - Timing Of Revenue Recognition Liabilities per Segment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Revenue [Line Items]
Total revenues	$ 1,167,319	$ 973,135	$ 2,271,357	$ 1,992,026
Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	1,002,743	879,194	1,981,172	1,789,814
Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	164,576	93,941	290,185	202,212
Research
Disaggregation of Revenue [Line Items]
Total revenues	1,003,238	875,329	1,982,970	1,784,620
Research | Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	916,754	810,564	1,810,841	1,639,776
Research | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	86,484	64,765	172,129	144,844
Conferences
Disaggregation of Revenue [Line Items]
Total revenues	58,179	317	82,981	14,187
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	58,179	317	82,981	14,187
Consulting
Disaggregation of Revenue [Line Items]
Total revenues	105,902	97,489	205,406	193,219
Consulting | Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	85,989	68,630	170,331	150,038
Consulting | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	$ 19,913	$ 28,859	$ 35,075	$ 43,181