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Revenue and Related Matters - Timing Of Revenue Recognition Liabilities per Segment (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
Transferred over time        
Disaggregation of Revenue [Line Items]        
Total revenues 1,021,744 897,647 3,002,916 2,687,461
Transferred at a point in time        
Disaggregation of Revenue [Line Items]        
Total revenues 134,538 96,971 424,723 299,183
Research        
Disaggregation of Revenue [Line Items]        
Total revenues 1,037,124 892,719 3,020,094 2,677,339
Research | Transferred over time        
Disaggregation of Revenue [Line Items]        
Total revenues 944,206 823,658 2,755,047 2,463,434
Research | Transferred at a point in time        
Disaggregation of Revenue [Line Items]        
Total revenues 92,918 69,061 265,047 213,905
Conferences        
Disaggregation of Revenue [Line Items]        
Total revenues 24,415 12,738 107,396 26,925
Conferences | Transferred over time        
Disaggregation of Revenue [Line Items]        
Total revenues 0 0 0 0
Conferences | Transferred at a point in time        
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
Consulting | Transferred over time        
Disaggregation of Revenue [Line Items]        
Total revenues 77,538 73,989 247,869 224,027
Consulting | Transferred at a point in time        
Disaggregation of Revenue [Line Items]        
Total revenues $ 17,205 $ 15,172 $ 52,280 $ 58,353