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, 2022	Dec. 31, 2021	Dec. 31, 2020
Segment Reporting Information [Line Items]
Total revenues	$ 537,787	$ 494,315	$ 448,984
Segment expenses	(212,256)	(182,634)	(162,232)
Selling, marketing, administrative and other expenses	(270,381)	(257,576)	(245,115)
Amortization of intangible assets	(13,161)	(15,129)	(19,683)
Restructuring costs	(9,335)	0	0
Integration costs	0	334	5,779
Interest expense, other income, and gains on investments	(1,930)	(5,451)	(3,242)
Income before income taxes	30,724	33,191	12,933
Research Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	396,012	372,587	351,950
Segment expenses	(133,566)	(118,155)	(110,843)
Consulting Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	111,028	108,867	86,897
Segment expenses	(56,889)	(51,770)	(40,168)
Events Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	30,747	12,861	10,137
Segment expenses	(21,801)	(12,709)	(11,221)
Research Revenues [Member]
Segment Reporting Information [Line Items]
Total revenues	354,453	325,340	301,544
Research Revenues [Member] | Research Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	354,453	325,340	301,544
Research Revenues [Member] | Consulting Segment [Member]
Segment Reporting Information [Line Items]
Total revenues			0
Research Revenues [Member] | Events Segment [Member]
Segment Reporting Information [Line Items]
Total revenues			0
Consulting [Member]
Segment Reporting Information [Line Items]
Total revenues	152,587	156,114	137,303
Consulting [Member] | Research Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	41,559	47,247	50,406
Consulting [Member] | Consulting Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	111,028	108,867	86,897
Consulting [Member] | Events Segment [Member]
Segment Reporting Information [Line Items]
Total revenues			0
Events [Member]
Segment Reporting Information [Line Items]
Total revenues	30,747	12,861	10,137
Events [Member] | Research Segment [Member]
Segment Reporting Information [Line Items]
Total revenues			0
Events [Member] | Consulting Segment [Member]
Segment Reporting Information [Line Items]
Total revenues			0
Events [Member] | Events Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	$ 30,747	$ 12,861	$ 10,137