Operating Segment and Enterprise Wide Reporting - Schedule of Information about Reportable Segments, Disaggregation of Revenue by Product (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Total revenues	$ 448,984	$ 461,697	$ 357,575
Segment expenses	(159,242)	(178,042)	(129,074)
Selling, marketing, administrative and other expenses	(248,105)	(253,163)	(201,127)
Amortization of intangible assets	(19,683)	(22,619)	(1,162)
Acquisition and integration costs	(5,779)	(8,948)	(3,787)
Interest expense, other expense, and gains on investments	(3,242)	(8,524)	1,100
Income (loss) before income taxes	12,933	(9,599)	23,525
Research Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	351,761	359,493	285,262
Segment expenses	(110,843)	(120,882)	(88,015)
Consulting Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	87,086	75,194	58,842
Segment expenses	(40,168)	(38,192)	(30,957)
Events Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	10,137	27,010	13,471
Segment expenses	(8,231)	(18,968)	(10,102)
Research Revenues [Member]
Segment Reporting Information [Line Items]
Total revenues	219,670	219,189	157,112
Research Revenues [Member] | Research Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	219,670	219,189	157,112
Connect Revenues [Member]
Segment Reporting Information [Line Items]
Total revenues	54,530	56,224	51,377
Connect Revenues [Member] | Research Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	54,530	56,224	51,377
Analytics Revenues [Member]
Segment Reporting Information [Line Items]
Total revenues	20,285	23,322	19,910
Analytics Revenues [Member] | Research Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	20,285	23,322	19,910
Total Research Revenues [Member]
Segment Reporting Information [Line Items]
Total revenues	294,485	298,735	228,399
Total Research Revenues [Member] | Research Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	294,485	298,735	228,399
Consulting [Member]
Segment Reporting Information [Line Items]
Total revenues	144,362	135,952	115,705
Consulting [Member] | Research Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	57,276	60,758	56,863
Consulting [Member] | Consulting Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	87,086	75,194	58,842
Events [Member]
Segment Reporting Information [Line Items]
Total revenues	10,137	27,010	13,471
Events [Member] | Events Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	$ 10,137	$ 27,010	$ 13,471