Revenue and Related Matters - Disaggregation of Revenue by Geographic Area (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
Research
Disaggregation of Revenue [Line Items]
Total revenues	979,732	909,291
Conferences
Disaggregation of Revenue [Line Items]
Total revenues	24,802	13,870
Consulting
Disaggregation of Revenue [Line Items]
Total revenues	99,504	95,730
United States and Canada
Disaggregation of Revenue [Line Items]
Total revenues	708,418	650,299
United States and Canada | Research
Disaggregation of Revenue [Line Items]
Total revenues	631,333	590,156
United States and Canada | Conferences
Disaggregation of Revenue [Line Items]
Total revenues	19,599	5,980
United States and Canada | Consulting
Disaggregation of Revenue [Line Items]
Total revenues	57,486	54,163
Europe, Middle East and Africa
Disaggregation of Revenue [Line Items]
Total revenues	263,276	238,168
Europe, Middle East and Africa | Research
Disaggregation of Revenue [Line Items]
Total revenues	230,201	205,939
Europe, Middle East and Africa | Conferences
Disaggregation of Revenue [Line Items]
Total revenues	2,713	2,147
Europe, Middle East and Africa | Consulting
Disaggregation of Revenue [Line Items]
Total revenues	30,362	30,082
Other International
Disaggregation of Revenue [Line Items]
Total revenues	132,344	130,424
Other International | Research
Disaggregation of Revenue [Line Items]
Total revenues	118,198	113,196
Other International | Conferences
Disaggregation of Revenue [Line Items]
Total revenues	2,490	5,743
Other International | Consulting
Disaggregation of Revenue [Line Items]
Total revenues	$ 11,656	$ 11,485