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Revenue and Related Matters - Disaggregation of Revenue by Geographic Area (Details) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2021 | Jun. 30, 2020 | Jun. 30, 2021 | Jun. 30, 2020
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenues | $ 1,167,319 | $ 973,135 | $ 2,271,357 | $ 1,992,026
Research |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenues | 1,003,238 | 875,329 | 1,982,970 | 1,784,620
Conferences |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenues | 58,179 | 317 | 82,981 | 14,187
Consulting |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenues | 105,902 | 97,489 | 205,406 | 193,219
United States and Canada |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenues | 746,365 | 631,579 | 1,454,783 | 1,281,878
United States and Canada | Research |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenues | 646,432 | 573,245 | 1,277,765 | 1,163,400
United States and Canada | Conferences |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenues | 40,328 | 317 | 59,927 | 6,297
United States and Canada | Consulting |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenues | 59,605 | 58,017 | 117,091 | 112,181
Europe, Middle East and Africa |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenues | 286,683 | 226,968 | 549,959 | 465,137
Europe, Middle East and Africa | Research |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenues | 236,756 | 196,854 | 466,957 | 402,794
Europe, Middle East and Africa | Conferences |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenues | 14,468 | 0 | 17,181 | 2,147
Europe, Middle East and Africa | Consulting |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenues | 35,459 | 30,114 | 65,821 | 60,196
Other International |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenues | 134,271 | 114,588 | 266,615 | 245,011
Other International | Research |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenues | 120,050 | 105,230 | 238,248 | 218,426
Other International | Conferences |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenues | 3,383 | 0 | 5,873 | 5,743
Other International | Consulting |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenues | $ 10,838 | $ 9,358 | $ 22,494 | $ 20,842

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