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Revenue and Related Matters - Disaggregation of Revenue by Geographic Area (Details) - USD ($) $ in Thousands | 12 Months Ended
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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

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