Revenue and Related Matters - Disaggregation of Revenue by Geographic Area (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Total revenues	$ 4,099,403	$ 4,245,321	$ 3,975,454
United States and Canada
Disaggregation of Revenue [Line Items]
Total revenues	2,637,824	2,734,490	2,514,952
Europe, Middle East and Africa
Disaggregation of Revenue [Line Items]
Total revenues	966,273	996,004	1,000,490
Other International
Disaggregation of Revenue [Line Items]
Total revenues	495,306	514,827	460,012
Research
Disaggregation of Revenue [Line Items]
Total revenues	3,602,892	3,374,548	3,105,764
Research | United States and Canada
Disaggregation of Revenue [Line Items]
Total revenues	2,339,482	2,199,008	1,994,016
Research | Europe, Middle East and Africa
Disaggregation of Revenue [Line Items]
Total revenues	826,752	751,267	737,129
Research | Other International
Disaggregation of Revenue [Line Items]
Total revenues	436,658	424,273	374,619
Conferences
Disaggregation of Revenue [Line Items]
Total revenues	120,140	476,869	410,461
Conferences | United States and Canada
Disaggregation of Revenue [Line Items]
Total revenues	75,024	295,857	256,219
Conferences | Europe, Middle East and Africa
Disaggregation of Revenue [Line Items]
Total revenues	28,108	122,591	105,909
Conferences | Other International
Disaggregation of Revenue [Line Items]
Total revenues	17,008	58,421	48,333
Consulting
Disaggregation of Revenue [Line Items]
Total revenues	376,371	393,904	353,667
Consulting | United States and Canada
Disaggregation of Revenue [Line Items]
Total revenues	223,318	239,625	205,874
Consulting | Europe, Middle East and Africa
Disaggregation of Revenue [Line Items]
Total revenues	111,413	122,146	119,258
Consulting | Other International
Disaggregation of Revenue [Line Items]
Total revenues	$ 41,640	$ 32,133	28,535
Other
Disaggregation of Revenue [Line Items]
Total revenues			105,562
Other | United States and Canada
Disaggregation of Revenue [Line Items]
Total revenues			58,843
Other | Europe, Middle East and Africa
Disaggregation of Revenue [Line Items]
Total revenues			38,194
Other | Other International
Disaggregation of Revenue [Line Items]
Total revenues			$ 8,525