Revenue and Related Matters - Disaggregation of Revenue by Geographic Area (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Total revenues	$ 994,618	$ 1,000,502	$ 2,986,644	$ 3,041,828
Research
Disaggregation of Revenue [Line Items]
Total revenues	892,719	840,998	2,677,339	2,492,427
Conferences
Disaggregation of Revenue [Line Items]
Total revenues	12,738	66,286	26,925	259,392
Consulting
Disaggregation of Revenue [Line Items]
Total revenues	89,161	93,218	282,380	290,009
United States and Canada
Disaggregation of Revenue [Line Items]
Total revenues	640,261	624,880	1,922,139	1,926,770
United States and Canada | Research
Disaggregation of Revenue [Line Items]
Total revenues	574,203	538,112	1,737,603	1,593,806
United States and Canada | Conferences
Disaggregation of Revenue [Line Items]
Total revenues	10,669	28,153	16,966	157,756
United States and Canada | Consulting
Disaggregation of Revenue [Line Items]
Total revenues	55,389	58,615	167,570	175,208
Europe, Middle East and Africa
Disaggregation of Revenue [Line Items]
Total revenues	235,711	247,086	700,848	741,612
Europe, Middle East and Africa | Research
Disaggregation of Revenue [Line Items]
Total revenues	210,152	196,121	612,946	583,742
Europe, Middle East and Africa | Conferences
Disaggregation of Revenue [Line Items]
Total revenues	2,064	24,497	4,211	67,520
Europe, Middle East and Africa | Consulting
Disaggregation of Revenue [Line Items]
Total revenues	23,495	26,468	83,691	90,350
Other International
Disaggregation of Revenue [Line Items]
Total revenues	118,646	128,536	363,657	373,446
Other International | Research
Disaggregation of Revenue [Line Items]
Total revenues	108,364	106,765	326,790	314,879
Other International | Conferences
Disaggregation of Revenue [Line Items]
Total revenues	5	13,636	5,748	34,116
Other International | Consulting
Disaggregation of Revenue [Line Items]
Total revenues	$ 10,277	$ 8,135	$ 31,119	$ 24,451