Operating Segments - Schedule of Information by Reportable Segment (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Reporting Information [Line Items]
Total revenues	$ 128,670	$ 113,541	$ 242,451	$ 219,886
Segment expenses	(45,300)	(39,175)	(89,056)	(77,337)
Selling, marketing, administrative and other expenses	(66,103)	(57,810)	(124,571)	(117,685)
Amortization of intangible assets	(3,968)	(4,713)	(7,871)	(9,425)
Integration costs	(216)	(612)	(334)	(3,487)
Interest expense, other income (expense) and gains (losses) on investments	(1,267)	844	(2,866)	(371)
Income before income taxes	11,816	12,075	17,753	11,581
Research Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	93,844	86,962	181,543	173,811
Segment expenses	(29,417)	(25,870)	(60,134)	(53,334)
Consulting Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	28,118	21,547	53,937	40,953
Segment expenses	(12,851)	(10,099)	(25,176)	(20,120)
Events Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	6,708	5,032	6,971	5,122
Segment expenses	(3,032)	(3,206)	(3,746)	(3,883)
Research [Member]
Segment Reporting Information [Line Items]
Total revenues	81,002	75,034	155,970	149,301
Research [Member] | Research Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	81,002	75,034	155,970	149,301
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	40,960	33,475	79,510	65,463
Consulting [Member] | Research Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	12,842	11,928	25,573	24,510
Consulting [Member] | Consulting Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	28,118	21,547	53,937	40,953
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	6,708	5,032	6,971	5,122
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	$ 6,708	$ 5,032	$ 6,971	$ 5,122