Revenue and Related Matters - Timing Of Revenue Recognition Liabilities per Segment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Disaggregation of Revenue [Line Items]
Total revenues	$ 1,104,038	$ 1,018,891
Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	978,429	910,620
Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	125,609	108,271
Research
Disaggregation of Revenue [Line Items]
Total revenues	979,732	909,291
Research | Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	894,087	829,212
Research | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	85,645	80,079
Conferences
Disaggregation of Revenue [Line Items]
Total revenues	24,802	13,870
Conferences | Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	0	0
Conferences | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	24,802	13,870
Consulting
Disaggregation of Revenue [Line Items]
Total revenues	99,504	95,730
Consulting | Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	84,342	81,408
Consulting | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	$ 15,162	$ 14,322