Revenue and Related Matters - Disaggregation of Revenue by Geographic Area (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Disaggregation of Revenue [Line Items]
Total revenues	$ 1,262,740	$ 1,104,038
Research
Disaggregation of Revenue [Line Items]
Total revenues	1,136,380	979,732
Conferences
Disaggregation of Revenue [Line Items]
Total revenues	10,354	24,802
Consulting
Disaggregation of Revenue [Line Items]
Total revenues	116,006	99,504
United States and Canada
Disaggregation of Revenue [Line Items]
Total revenues	816,970	708,418
United States and Canada | Research
Disaggregation of Revenue [Line Items]
Total revenues	740,529	631,333
United States and Canada | Conferences
Disaggregation of Revenue [Line Items]
Total revenues	7,652	19,599
United States and Canada | Consulting
Disaggregation of Revenue [Line Items]
Total revenues	68,789	57,486
Europe, Middle East and Africa
Disaggregation of Revenue [Line Items]
Total revenues	296,811	263,276
Europe, Middle East and Africa | Research
Disaggregation of Revenue [Line Items]
Total revenues	263,129	230,201
Europe, Middle East and Africa | Conferences
Disaggregation of Revenue [Line Items]
Total revenues	1,238	2,713
Europe, Middle East and Africa | Consulting
Disaggregation of Revenue [Line Items]
Total revenues	32,444	30,362
Other International
Disaggregation of Revenue [Line Items]
Total revenues	148,959	132,344
Other International | Research
Disaggregation of Revenue [Line Items]
Total revenues	132,722	118,198
Other International | Conferences
Disaggregation of Revenue [Line Items]
Total revenues	1,464	2,490
Other International | Consulting
Disaggregation of Revenue [Line Items]
Total revenues	$ 14,773	$ 11,656