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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