Business and Basis of Presentation - Disaggregation of Revenue by Geographic Area (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Total revenues	$ 921,674	$ 828,085	$ 2,886,575	$ 2,296,985
Research
Disaggregation of Revenue [Line Items]
Total revenues	774,188	653,443	2,308,426	1,778,481
Events
Disaggregation of Revenue [Line Items]
Total revenues	57,141	44,953	214,481	171,427
Consulting
Disaggregation of Revenue [Line Items]
Total revenues	78,752	72,117	258,106	242,404
Other
Disaggregation of Revenue [Line Items]
Total revenues	11,593	57,572	105,562	104,673
United States and Canada
Disaggregation of Revenue [Line Items]
Total revenues	579,342	493,350	1,822,995	1,426,401
United States and Canada | Research
Disaggregation of Revenue [Line Items]
Total revenues	502,489	406,732	1,485,545	1,127,039
United States and Canada | Events
Disaggregation of Revenue [Line Items]
Total revenues	22,002	19,177	131,215	105,740
United States and Canada | Consulting
Disaggregation of Revenue [Line Items]
Total revenues	45,903	36,483	146,816	139,266
United States and Canada | Other
Disaggregation of Revenue [Line Items]
Total revenues	8,948	30,958	59,419	54,356
Europe, Middle East and Africa
Disaggregation of Revenue [Line Items]
Total revenues	234,446	242,346	736,385	608,330
Europe, Middle East and Africa | Research
Disaggregation of Revenue [Line Items]
Total revenues	180,124	176,459	551,070	445,039
Europe, Middle East and Africa | Events
Disaggregation of Revenue [Line Items]
Total revenues	25,422	18,098	57,163	45,052
Europe, Middle East and Africa | Consulting
Disaggregation of Revenue [Line Items]
Total revenues	26,255	29,064	89,783	82,554
Europe, Middle East and Africa | Other
Disaggregation of Revenue [Line Items]
Total revenues	2,645	18,725	38,369	35,685
Other International
Disaggregation of Revenue [Line Items]
Total revenues	107,886	92,389	327,195	262,254
Other International | Research
Disaggregation of Revenue [Line Items]
Total revenues	91,575	70,252	271,811	206,403
Other International | Events
Disaggregation of Revenue [Line Items]
Total revenues	9,717	7,678	26,103	20,635
Other International | Consulting
Disaggregation of Revenue [Line Items]
Total revenues	6,594	6,570	21,507	20,584
Other International | Other
Disaggregation of Revenue [Line Items]
Total revenues	$ 0	$ 7,889	$ 7,774	$ 14,632