Business and Basis of Presentation - Disaggregation of Revenue by Geographic Area (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation of Revenue [Line Items]
Total revenues	$ 1,001,336	$ 843,731	$ 1,964,901	$ 1,468,900
Research
Disaggregation of Revenue [Line Items]
Total revenues	770,314	613,732	1,534,238	1,125,038
Events
Disaggregation of Revenue [Line Items]
Total revenues	111,253	91,205	157,340	126,474
Consulting
Disaggregation of Revenue [Line Items]
Total revenues	96,458	91,693	179,354	170,287
Other
Disaggregation of Revenue [Line Items]
Total revenues	23,311	47,101	93,969	47,101
United States and Canada
Disaggregation of Revenue [Line Items]
Total revenues	650,148	540,064	1,243,653	933,051
United States and Canada | Research
Disaggregation of Revenue [Line Items]
Total revenues	493,343	392,815	983,056	720,307
United States and Canada | Events
Disaggregation of Revenue [Line Items]
Total revenues	85,144	68,900	109,213	86,563
United States and Canada | Consulting
Disaggregation of Revenue [Line Items]
Total revenues	55,784	54,951	100,913	102,783
United States and Canada | Other
Disaggregation of Revenue [Line Items]
Total revenues	15,877	23,398	50,471	23,398
Europe, Middle East and Africa
Disaggregation of Revenue [Line Items]
Total revenues	242,273	212,264	501,939	365,984
Europe, Middle East and Africa | Research
Disaggregation of Revenue [Line Items]
Total revenues	186,399	152,778	370,946	268,580
Europe, Middle East and Africa | Events
Disaggregation of Revenue [Line Items]
Total revenues	14,850	12,859	31,741	26,954
Europe, Middle East and Africa | Consulting
Disaggregation of Revenue [Line Items]
Total revenues	33,590	29,667	63,528	53,490
Europe, Middle East and Africa | Other
Disaggregation of Revenue [Line Items]
Total revenues	7,434	16,960	35,724	16,960
Other International
Disaggregation of Revenue [Line Items]
Total revenues	108,915	91,403	219,309	169,865
Other International | Research
Disaggregation of Revenue [Line Items]
Total revenues	90,572	68,139	180,236	136,151
Other International | Events
Disaggregation of Revenue [Line Items]
Total revenues	11,259	9,446	16,386	12,957
Other International | Consulting
Disaggregation of Revenue [Line Items]
Total revenues	7,084	7,075	14,913	14,014
Other International | Other
Disaggregation of Revenue [Line Items]
Total revenues	$ 0	$ 6,743	$ 7,774	$ 6,743