Revenue and Related Matters - Disaggregation of Revenue by Geographic Area (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Disaggregation of Revenue [Line Items]
Total revenues	$ 4,733,962	$ 4,099,403	$ 4,245,321
United States and Canada
Disaggregation of Revenue [Line Items]
Total revenues	3,048,902	2,637,824	2,734,490
Europe, Middle East and Africa
Disaggregation of Revenue [Line Items]
Total revenues	1,130,979	966,273	996,004
Other International
Disaggregation of Revenue [Line Items]
Total revenues	554,081	495,306	514,827
Research
Disaggregation of Revenue [Line Items]
Total revenues	4,101,392	3,602,892	3,374,548
Research | United States and Canada
Disaggregation of Revenue [Line Items]
Total revenues	2,655,534	2,339,482	2,199,008
Research | Europe, Middle East and Africa
Disaggregation of Revenue [Line Items]
Total revenues	958,339	826,752	751,267
Research | Other International
Disaggregation of Revenue [Line Items]
Total revenues	487,519	436,658	424,273
Conferences
Disaggregation of Revenue [Line Items]
Total revenues	214,449	120,140	476,869
Conferences | United States and Canada
Disaggregation of Revenue [Line Items]
Total revenues	146,707	75,024	295,857
Conferences | Europe, Middle East and Africa
Disaggregation of Revenue [Line Items]
Total revenues	47,883	28,108	122,591
Conferences | Other International
Disaggregation of Revenue [Line Items]
Total revenues	19,859	17,008	58,421
Consulting
Disaggregation of Revenue [Line Items]
Total revenues	418,121	376,371	393,904
Consulting | United States and Canada
Disaggregation of Revenue [Line Items]
Total revenues	246,661	223,318	239,625
Consulting | Europe, Middle East and Africa
Disaggregation of Revenue [Line Items]
Total revenues	124,757	111,413	122,146
Consulting | Other International
Disaggregation of Revenue [Line Items]
Total revenues	$ 46,703	$ 41,640	$ 32,133