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Revenue and Related Matters - Disaggregation of Revenue by Geographic Area (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenues | $ 1,156,282 | $ 994,618 | $ 3,427,639 | $ 2,986,644
Research |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenues | 1,037,124 | 892,719 | 3,020,094 | 2,677,339
Conferences |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenues | 24,415 | 12,738 | 107,396 | 26,925
Consulting |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenues | 94,743 | 89,161 | 300,149 | 282,380
United States and Canada |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenues | 742,403 | 640,261 | 2,197,186 | 1,922,139
United States and Canada | Research |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenues | 671,517 | 574,203 | 1,949,282 | 1,737,603
United States and Canada | Conferences |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenues | 14,171 | 10,669 | 74,098 | 16,966
United States and Canada | Consulting |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenues | 56,715 | 55,389 | 173,806 | 167,570
Europe, Middle East and Africa |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenues | 276,955 | 235,711 | 826,914 | 700,848
Europe, Middle East and Africa | Research |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenues | 242,273 | 210,152 | 709,230 | 612,946
Europe, Middle East and Africa | Conferences |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenues | 7,975 | 2,064 | 25,156 | 4,211
Europe, Middle East and Africa | Consulting |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenues | 26,707 | 23,495 | 92,528 | 83,691
Other International |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenues | 136,924 | 118,646 | 403,539 | 363,657
Other International | Research |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenues | 123,334 | 108,364 | 361,582 | 326,790
Other International | Conferences |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenues | 2,269 | 5 | 8,142 | 5,748
Other International | Consulting |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenues | $ 11,321 | $ 10,277 | $ 33,815 | $ 31,119

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