Business and Basis of Presentation - Disaggregation of Revenue by Geographic Area (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Total revenues	$ 970,444	$ 963,565
Research
Disaggregation of Revenue [Line Items]
Total revenues	825,374	763,924
Conferences
Disaggregation of Revenue [Line Items]
Total revenues	51,932	46,087
Consulting
Disaggregation of Revenue [Line Items]
Total revenues	93,138	82,896
Other
Disaggregation of Revenue [Line Items]
Total revenues		70,658
United States and Canada
Disaggregation of Revenue [Line Items]
Total revenues	611,333	593,505
United States and Canada | Research
Disaggregation of Revenue [Line Items]
Total revenues	527,233	489,713
United States and Canada | Conferences
Disaggregation of Revenue [Line Items]
Total revenues	29,007	24,069
United States and Canada | Consulting
Disaggregation of Revenue [Line Items]
Total revenues	55,093	45,129
United States and Canada | Other
Disaggregation of Revenue [Line Items]
Total revenues		34,594
Europe, Middle East and Africa
Disaggregation of Revenue [Line Items]
Total revenues	241,086	259,666
Europe, Middle East and Africa | Research
Disaggregation of Revenue [Line Items]
Total revenues	193,955	184,547
Europe, Middle East and Africa | Conferences
Disaggregation of Revenue [Line Items]
Total revenues	17,197	16,891
Europe, Middle East and Africa | Consulting
Disaggregation of Revenue [Line Items]
Total revenues	29,934	29,938
Europe, Middle East and Africa | Other
Disaggregation of Revenue [Line Items]
Total revenues		28,290
Other International
Disaggregation of Revenue [Line Items]
Total revenues	118,025	110,394
Other International | Research
Disaggregation of Revenue [Line Items]
Total revenues	104,186	89,664
Other International | Conferences
Disaggregation of Revenue [Line Items]
Total revenues	5,728	5,127
Other International | Consulting
Disaggregation of Revenue [Line Items]
Total revenues	$ 8,111	7,829
Other International | Other
Disaggregation of Revenue [Line Items]
Total revenues		$ 7,774