Operating Segment and Enterprise Wide Reporting - Schedule of Information by Reportable Segment (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Segment Reporting Information [Line Items]
Total revenues	$ 480,779	$ 537,787	$ 494,315
Segment expenses	(198,029)	(212,256)	(182,634)
Selling, marketing, administrative and other expenses	(250,756)	(270,381)	(257,576)
Amortization of intangible assets	(11,956)	(13,161)	(15,129)
Restructuring costs	(13,272)	(9,335)	0
Integration costs	0	0	(334)
Interest expense, other income, and gains on investments	(481)	(1,930)	(5,451)
Income before income taxes	6,285	30,724	33,191
Research Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	363,222	396,012	372,587
Segment expenses	(132,444)	(133,566)	(118,155)
Consulting Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	89,402	111,028	108,867
Segment expenses	(45,028)	(56,889)	(51,770)
Events Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	28,155	30,747	12,861
Segment expenses	(20,557)	(21,801)	(12,709)
Research [Member]
Segment Reporting Information [Line Items]
Total revenues	334,396	354,453	325,340
Research [Member] | Research Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	334,396	354,453	325,340
Research [Member] | Consulting Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	0	0	0
Research [Member] | Events Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	0	0	0
Consulting [Member]
Segment Reporting Information [Line Items]
Total revenues	118,228	152,587	156,114
Consulting [Member] | Research Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	28,826	41,559	47,247
Consulting [Member] | Consulting Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	89,402	111,028	108,867
Consulting [Member] | Events Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	0	0	0
Events [Member]
Segment Reporting Information [Line Items]
Total revenues	28,155	30,747	12,861
Events [Member] | Research Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	0	0	0
Events [Member] | Consulting Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	0	0	0
Events [Member] | Events Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	$ 28,155	$ 30,747	$ 12,861