Revenue and Related Matters - Disaggregation of Revenue by Geographic Area (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Total revenues	$ 4,245,321	$ 3,975,454	$ 3,311,494
United States and Canada
Disaggregation of Revenue [Line Items]
Total revenues	2,734,490	2,514,952	2,092,366
Europe, Middle East and Africa
Disaggregation of Revenue [Line Items]
Total revenues	996,004	1,000,490	855,421
Other International
Disaggregation of Revenue [Line Items]
Total revenues	514,827	460,012	363,707
Research
Disaggregation of Revenue [Line Items]
Total revenues	3,374,548	3,105,764	2,471,280
Research | United States and Canada
Disaggregation of Revenue [Line Items]
Total revenues	2,199,008	1,994,016	1,600,847
Research | Europe, Middle East and Africa
Disaggregation of Revenue [Line Items]
Total revenues	751,267	737,129	597,943
Research | Other International
Disaggregation of Revenue [Line Items]
Total revenues	424,273	374,619	272,490
Conferences
Disaggregation of Revenue [Line Items]
Total revenues	476,869	410,461	337,903
Conferences | United States and Canada
Disaggregation of Revenue [Line Items]
Total revenues	295,857	256,219	210,698
Conferences | Europe, Middle East and Africa
Disaggregation of Revenue [Line Items]
Total revenues	122,591	105,909	86,567
Conferences | Other International
Disaggregation of Revenue [Line Items]
Total revenues	58,421	48,333	40,638
Consulting
Disaggregation of Revenue [Line Items]
Total revenues	393,904	353,667	327,661
Consulting | United States and Canada
Disaggregation of Revenue [Line Items]
Total revenues	239,625	205,874	188,022
Consulting | Europe, Middle East and Africa
Disaggregation of Revenue [Line Items]
Total revenues	122,146	119,258	111,792
Consulting | Other International
Disaggregation of Revenue [Line Items]
Total revenues	$ 32,133	28,535	27,847
Other
Disaggregation of Revenue [Line Items]
Total revenues		105,562	174,650
Other | United States and Canada
Disaggregation of Revenue [Line Items]
Total revenues		58,843	92,799
Other | Europe, Middle East and Africa
Disaggregation of Revenue [Line Items]
Total revenues		38,194	59,119
Other | Other International
Disaggregation of Revenue [Line Items]
Total revenues		$ 8,525	$ 22,732