Business and Basis of Presentation - Disaggregation of Revenue by Geographic Area (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Total revenues	$ 1,070,882	$ 1,001,336	$ 2,041,326	$ 1,964,901
Research
Disaggregation of Revenue [Line Items]
Total revenues	826,055	770,314	1,651,429	1,534,238
Conferences
Disaggregation of Revenue [Line Items]
Total revenues	141,174	111,253	193,106	157,340
Consulting
Disaggregation of Revenue [Line Items]
Total revenues	103,653	96,458	196,791	179,354
Other
Disaggregation of Revenue [Line Items]
Total revenues		23,311		93,969
United States and Canada
Disaggregation of Revenue [Line Items]
Total revenues	690,556	650,148	1,301,889	1,243,653
United States and Canada | Research
Disaggregation of Revenue [Line Items]
Total revenues	528,461	493,343	1,055,694	983,056
United States and Canada | Conferences
Disaggregation of Revenue [Line Items]
Total revenues	100,596	85,144	129,603	109,213
United States and Canada | Consulting
Disaggregation of Revenue [Line Items]
Total revenues	61,499	55,784	116,592	100,913
United States and Canada | Other
Disaggregation of Revenue [Line Items]
Total revenues		15,877		50,471
Europe, Middle East and Africa
Disaggregation of Revenue [Line Items]
Total revenues	253,441	242,273	494,527	501,939
Europe, Middle East and Africa | Research
Disaggregation of Revenue [Line Items]
Total revenues	193,666	186,399	387,621	370,946
Europe, Middle East and Africa | Conferences
Disaggregation of Revenue [Line Items]
Total revenues	25,827	14,850	43,024	31,741
Europe, Middle East and Africa | Consulting
Disaggregation of Revenue [Line Items]
Total revenues	33,948	33,590	63,882	63,528
Europe, Middle East and Africa | Other
Disaggregation of Revenue [Line Items]
Total revenues		7,434		35,724
Other International
Disaggregation of Revenue [Line Items]
Total revenues	126,885	108,915	244,910	219,309
Other International | Research
Disaggregation of Revenue [Line Items]
Total revenues	103,928	90,572	208,114	180,236
Other International | Conferences
Disaggregation of Revenue [Line Items]
Total revenues	14,751	11,259	20,479	16,386
Other International | Consulting
Disaggregation of Revenue [Line Items]
Total revenues	$ 8,206	7,084	$ 16,317	14,913
Other International | Other
Disaggregation of Revenue [Line Items]
Total revenues		$ 0		$ 7,774