Revenue and Related Matters - Disaggregation of Revenue by Geographic Area (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Total revenues	$ 1,018,891	$ 970,444
Research
Disaggregation of Revenue [Line Items]
Total revenues	909,291	825,374
Conferences
Disaggregation of Revenue [Line Items]
Total revenues	13,870	51,932
Consulting
Disaggregation of Revenue [Line Items]
Total revenues	95,730	93,138
United States and Canada
Disaggregation of Revenue [Line Items]
Total revenues	650,299	611,333
United States and Canada | Research
Disaggregation of Revenue [Line Items]
Total revenues	590,156	527,233
United States and Canada | Conferences
Disaggregation of Revenue [Line Items]
Total revenues	5,980	29,007
United States and Canada | Consulting
Disaggregation of Revenue [Line Items]
Total revenues	54,163	55,093
Europe, Middle East and Africa
Disaggregation of Revenue [Line Items]
Total revenues	238,168	241,086
Europe, Middle East and Africa | Research
Disaggregation of Revenue [Line Items]
Total revenues	205,939	193,955
Europe, Middle East and Africa | Conferences
Disaggregation of Revenue [Line Items]
Total revenues	2,147	17,197
Europe, Middle East and Africa | Consulting
Disaggregation of Revenue [Line Items]
Total revenues	30,082	29,934
Other International
Disaggregation of Revenue [Line Items]
Total revenues	130,424	118,025
Other International | Research
Disaggregation of Revenue [Line Items]
Total revenues	113,196	104,186
Other International | Conferences
Disaggregation of Revenue [Line Items]
Total revenues	5,743	5,728
Other International | Consulting
Disaggregation of Revenue [Line Items]
Total revenues	$ 11,485	$ 8,111