Operating Segments - Schedule of Information about Reportable Segments, Disaggregation of Revenue by Product (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Total revenues	$ 108,577	$ 108,596	$ 328,463	$ 337,428
Segment expenses	(39,575)	(39,664)	(116,912)	(131,385)
Selling, marketing, administrative and other expenses	(63,778)	(62,524)	(181,463)	(189,134)
Amortization of intangible assets	(4,722)	(5,654)	(14,147)	(16,963)
Acquisition and integration costs	(328)	(2,394)	(3,815)	(7,848)
Interest expense, other income (expense) and gains (losses) on investments	(1,533)	(1,794)	(1,904)	(6,631)
Income (loss) before income taxes	(1,359)	(3,434)	10,222	(14,533)
Research Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	86,185	87,905	259,979	262,204
Segment expenses	(28,645)	(28,632)	(81,979)	(89,780)
Consulting Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	21,261	19,262	62,231	55,654
Segment expenses	(9,646)	(9,299)	(29,766)	(28,207)
Events Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	1,131	1,429	6,253	19,570
Segment expenses	(1,284)	(1,733)	(5,167)	(13,398)
Research [Member]
Segment Reporting Information [Line Items]
Total revenues	54,107	55,022	163,071	161,107
Research [Member] | Research Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	54,107	55,022	163,071	161,107
Connect [Member]
Segment Reporting Information [Line Items]
Total revenues	13,710	14,257	40,677	41,895
Connect [Member] | Research Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	13,710	14,257	40,677	41,895
Analytics [Member]
Segment Reporting Information [Line Items]
Total revenues	4,996	5,269	15,482	16,434
Analytics [Member] | Research Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	4,996	5,269	15,482	16,434
Total Research Revenues [Member]
Segment Reporting Information [Line Items]
Total revenues	72,813	74,548	219,230	219,436
Total Research Revenues [Member] | Research Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	72,813	74,548	219,230	219,436
Consulting [Member]
Segment Reporting Information [Line Items]
Total revenues	34,633	32,619	102,980	98,422
Consulting [Member] | Research Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	13,372	13,357	40,749	42,768
Consulting [Member] | Consulting Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	21,261	19,262	62,231	55,654
Events [Member]
Segment Reporting Information [Line Items]
Total revenues	1,131	1,429	6,253	19,570
Events [Member] | Events Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	$ 1,131	$ 1,429	$ 6,253	$ 19,570