Operating Segments - Schedule of Information by Reportable Segment (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Segment Reporting Information [Line Items]
Total revenues	$ 148,246	$ 128,670	$ 273,217	$ 242,451
Segment expenses	(58,007)	(46,199)	(108,255)	(90,805)
Selling, marketing, administrative and other expenses	(66,182)	(65,204)	(131,072)	(122,822)
Amortization of intangible assets	(3,354)	(3,968)	(6,716)	(7,871)
Integration costs	0	(216)	0	(334)
Interest expense, other income, and gains on investments	(432)	(1,267)	(876)	(2,866)
Income before income taxes	20,271	11,816	26,298	17,753
Research Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	100,368	93,844	197,338	181,543
Segment expenses	(32,897)	(29,417)	(67,077)	(60,134)
Consulting Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	28,341	28,118	55,582	53,937
Segment expenses	(14,059)	(12,851)	(28,376)	(25,176)
Events Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	19,537	6,708	20,297	6,971
Segment expenses	(11,051)	(3,931)	(12,802)	(5,495)
Research [Member]
Segment Reporting Information [Line Items]
Total revenues	89,447	81,002	175,227	155,970
Research [Member] | Research Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	89,447	81,002	175,227	155,970
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	39,262	40,960	77,693	79,510
Consulting [Member] | Research Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	10,921	12,842	22,111	25,573
Consulting [Member] | Consulting Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	28,341	28,118	55,582	53,937
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	19,537	6,708	20,297	6,971
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	$ 19,537	$ 6,708	$ 20,297	$ 6,971