Business and Basis of Presentation - Disaggregation of Revenue by Geographic Area (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Total revenues	$ 1,000,502	$ 921,674	$ 3,041,828	$ 2,886,575
Research
Disaggregation of Revenue [Line Items]
Total revenues	840,998	774,188	2,492,427	2,308,426
Conferences
Disaggregation of Revenue [Line Items]
Total revenues	66,286	57,141	259,392	214,481
Consulting
Disaggregation of Revenue [Line Items]
Total revenues	93,218	78,752	290,009	258,106
Other
Disaggregation of Revenue [Line Items]
Total revenues		11,593		105,562
United States and Canada
Disaggregation of Revenue [Line Items]
Total revenues	624,880	579,342	1,926,770	1,822,995
United States and Canada | Research
Disaggregation of Revenue [Line Items]
Total revenues	538,112	502,489	1,593,806	1,485,545
United States and Canada | Conferences
Disaggregation of Revenue [Line Items]
Total revenues	28,153	22,002	157,756	131,215
United States and Canada | Consulting
Disaggregation of Revenue [Line Items]
Total revenues	58,615	45,903	175,208	146,816
United States and Canada | Other
Disaggregation of Revenue [Line Items]
Total revenues		8,948		59,419
Europe, Middle East and Africa
Disaggregation of Revenue [Line Items]
Total revenues	247,086	234,446	741,612	736,385
Europe, Middle East and Africa | Research
Disaggregation of Revenue [Line Items]
Total revenues	196,121	180,124	583,742	551,070
Europe, Middle East and Africa | Conferences
Disaggregation of Revenue [Line Items]
Total revenues	24,497	25,422	67,520	57,163
Europe, Middle East and Africa | Consulting
Disaggregation of Revenue [Line Items]
Total revenues	26,468	26,255	90,350	89,783
Europe, Middle East and Africa | Other
Disaggregation of Revenue [Line Items]
Total revenues		2,645		38,369
Other International
Disaggregation of Revenue [Line Items]
Total revenues	128,536	107,886	373,446	327,195
Other International | Research
Disaggregation of Revenue [Line Items]
Total revenues	106,765	91,575	314,879	271,811
Other International | Conferences
Disaggregation of Revenue [Line Items]
Total revenues	13,636	9,717	34,116	26,103
Other International | Consulting
Disaggregation of Revenue [Line Items]
Total revenues	$ 8,135	6,594	$ 24,451	21,507
Other International | Other
Disaggregation of Revenue [Line Items]
Total revenues		$ 0		$ 7,774