Operating Segments - Schedule of Information by Reportable Segment (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Segment Reporting Information [Line Items]
Total revenues	$ 127,679	$ 118,136	$ 400,896	$ 360,587
Segment expenses	(51,182)	(44,606)	(159,437)	(135,411)
Selling, marketing, administrative and other expenses	(64,588)	(63,295)	(195,660)	(186,117)
Amortization of intangible assets	(3,352)	(3,696)	(10,068)	(11,567)
Integration costs	0	0	0	(334)
Interest expense, other income (expense), and gains on investments	(238)	(1,251)	(1,114)	(4,117)
Income before income taxes	8,319	5,288	34,617	23,041
Research Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	96,053	90,463	293,391	272,006
Segment expenses	(33,058)	(28,657)	(100,135)	(88,791)
Consulting Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	28,367	26,806	83,949	80,743
Segment expenses	(14,747)	(13,061)	(43,123)	(38,237)
Events Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	3,259	867	23,556	7,838
Segment expenses	(3,377)	(2,888)	(16,179)	(8,383)
Research [Member]
Segment Reporting Information [Line Items]
Total revenues	87,038	79,876	262,265	235,846
Research [Member] | Research Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	87,038	79,876	262,265	235,846
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	37,382	37,393	115,075	116,903
Consulting [Member] | Research Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	9,015	10,587	31,126	36,160
Consulting [Member] | Consulting Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	28,367	26,806	83,949	80,743
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	3,259	867	23,556	7,838
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	$ 3,259	$ 867	$ 23,556	$ 7,838