Operating Segments - Schedule of Information by Reportable Segment (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Segment Reporting Information [Line Items]
Total revenues	$ 121,825	$ 135,589	$ 221,902	$ 249,259
Segment expenses	(51,993)	(54,489)	(97,090)	(103,980)
Selling, marketing, administrative and other expenses	(55,923)	(59,212)	(111,057)	(123,876)
Amortization of intangible assets	(2,513)	(3,068)	(5,027)	(6,134)
Restructuring and related costs	(82)	(10,532)	(6,706)	(12,121)
Interest expense and other income	252	(216)	764	(459)
Income before income taxes	11,566	8,072	2,786	2,689
Research Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	89,163	94,702	170,387	183,527
Segment expenses	(31,974)	(32,536)	(65,488)	(68,043)
Consulting Segment Member
Segment Reporting Information [Line Items]
Total revenues	19,311	22,967	37,809	46,798
Segment expenses	(9,504)	(11,429)	(19,601)	(23,782)
Events Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	13,351	17,920	13,706	18,934
Segment expenses	(10,515)	(10,524)	(12,001)	(12,155)
Research [Member]
Segment Reporting Information [Line Items]
Total revenues	83,663	87,699	160,244	168,605
Research [Member] | Research Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	83,663	87,699	160,244	168,605
Research [Member] | Consulting Segment Member
Segment Reporting Information [Line Items]
Total revenues	0	0	0	0
Research [Member] | Events Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	0	0	0	0
Consulting [Member]
Segment Reporting Information [Line Items]
Total revenues	24,811	29,970	47,952	61,720
Consulting [Member] | Research Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	5,500	7,003	10,143	14,922
Consulting [Member] | Consulting Segment Member
Segment Reporting Information [Line Items]
Total revenues	19,311	22,967	37,809	46,798
Consulting [Member] | Events Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	0	0	0	0
Events [Member]
Segment Reporting Information [Line Items]
Total revenues	13,351	17,920	13,706	18,934
Events [Member] | Research Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	0	0	0	0
Events [Member] | Consulting Segment Member
Segment Reporting Information [Line Items]
Total revenues	0	0	0	0
Events [Member] | Events Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	$ 13,351	$ 17,920	$ 13,706	$ 18,934