Operating Segments - Schedule of Information by Reportable Segment (Detail) - USD ($) $ in Thousands	3 Months Ended					6 Months Ended
	Jun. 30, 2025		Mar. 31, 2025	Jun. 30, 2024		Jun. 30, 2025		Jun. 30, 2024
Segment Reporting Information [Line Items]
Total revenues	$ 111,659			$ 121,825		$ 201,535		$ 221,902
Income (loss) from operations	6,956			11,314		(80,635)		2,022
Selling, marketing, administrative and other expenses	(55,718)			(57,424)		(109,767)		(114,047)
Amortization of intangible assets	(2,217)			(2,513)		(4,434)		(5,027)
Goodwill impairment	0		$ (83,900)	0		(83,895)		0
Restructuring and related costs	(491)			(82)		(1,998)		(6,706)
Interest expense and other income				252				764
Interest expense, other income and loss on investments	160					(551)
Income (Loss) before income taxes	7,116			11,566		(81,186)		2,786
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	111,659			121,825		201,535		221,902
Segment expenses	(46,277)	[1]		(50,492)	[2]	(82,076)	[3]	(94,100)	[4]
Income (loss) from operations	65,382			71,333		119,459		127,802
Research Segment [Member]
Segment Reporting Information [Line Items]
Goodwill impairment						(83,895)
Research Segment [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	83,715			89,163		157,187		170,387
Segment expenses	(26,182)	[1]		(30,456)	[2]	(52,318)	[3]	(62,400)	[4]
Income (loss) from operations	57,533			58,707		104,869		107,987
Consulting Segment Member
Segment Reporting Information [Line Items]
Goodwill impairment						0
Consulting Segment Member | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	17,704			19,311		34,082		37,809
Segment expenses	(10,156)	[1]		(10,146)	[2]	(19,055)	[3]	(21,032)	[4]
Income (loss) from operations	7,548			9,165		15,027		16,777
Events Segment [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	10,240			13,351		10,266		13,706
Segment expenses	(9,939)	[1]		(9,890)	[2]	(10,703)	[3]	(10,668)	[4]
Income (loss) from operations	301			3,461		(437)		3,038
Research [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	77,926			83,663		146,340		160,244
Research [Member] | Research Segment [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	77,926			83,663		146,340		160,244
Research [Member] | Consulting Segment Member | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	0			0		0		0
Research [Member] | Events Segment [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	0			0		0		0
Consulting [Member]
Segment Reporting Information [Line Items]
Total revenues	23,493			24,811		44,929		47,952
Consulting [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	23,493			24,811		44,929		47,952
Consulting [Member] | Research Segment [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	5,789			5,500		10,847		10,143
Consulting [Member] | Consulting Segment Member | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	17,704			19,311		34,082		37,809
Consulting [Member] | Events Segment [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	0			0		0		0
Events [Member]
Segment Reporting Information [Line Items]
Total revenues	10,240			13,351		10,266		13,706
Events [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	10,240			13,351		10,266		13,706
Events [Member] | Research Segment [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	0			0		0		0
Events [Member] | Consulting Segment Member | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	0			0		0		0
Events [Member] | Events Segment [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	10,240			13,351		10,266		13,706
Compensation Benefits and Related Costs [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Segment expenses	(31,501)	[1]		(34,285)	[2]	(61,687)	[3]	(70,701)	[4]
Compensation Benefits and Related Costs [Member] | Research Segment [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Segment expenses	(23,414)	[1]		(25,898)	[2]	(45,966)	[3]	(53,161)	[4]
Compensation Benefits and Related Costs [Member] | Consulting Segment Member | Operating Segments [Member]
Segment Reporting Information [Line Items]
Segment expenses	(7,256)	[1]		(7,489)	[2]	(14,163)	[3]	(15,869)	[4]
Compensation Benefits and Related Costs [Member] | Events Segment [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Segment expenses	(831)	[1]		(898)	[2]	(1,558)	[3]	(1,671)	[4]
Direct Costs of Events [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Segment expenses	(8,973)	[1]		(8,934)	[2]	(8,973)	[3]	(8,934)	[4]
Direct Costs of Events [Member] | Research Segment [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Segment expenses	0	[1]		0	[2]	0	[3]	0	[4]
Direct Costs of Events [Member] | Consulting Segment Member | Operating Segments [Member]
Segment Reporting Information [Line Items]
Segment expenses	0	[1]		0	[2]	0	[3]	0	[4]
Direct Costs of Events [Member] | Events Segment [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Segment expenses	(8,973)	[1]		(8,934)	[2]	(8,973)	[3]	(8,934)	[4]
Professional Services [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Segment expenses	(2,744)	[1]		(3,518)	[2]	(5,949)	[3]	(7,650)	[4]
Professional Services [Member] | Research Segment [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Segment expenses	(1,657)	[1]		(3,110)	[2]	(4,348)	[3]	(6,761)	[4]
Professional Services [Member] | Consulting Segment Member | Operating Segments [Member]
Segment Reporting Information [Line Items]
Segment expenses	(1,010)	[1]		(393)	[2]	(1,523)	[3]	(871)	[4]
Professional Services [Member] | Events Segment [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Segment expenses	(77)	[1]		(15)	[2]	(78)	[3]	(18)	[4]
Billable Expenses [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Segment expenses	(1,858)	[1]		(2,323)	[2]	(3,298)	[3]	(4,411)	[4]
Billable Expenses [Member] | Research Segment [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Segment expenses	(102)	[1]		(144)	[2]	(185)	[3]	(292)	[4]
Billable Expenses [Member] | Consulting Segment Member | Operating Segments [Member]
Segment Reporting Information [Line Items]
Segment expenses	(1,756)	[1]		(2,179)	[2]	(3,113)	[3]	(4,119)	[4]
Billable Expenses [Member] | Events Segment [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Segment expenses	0	[1]		0	[2]	0	[3]	0	[4]
Travel and Entertainment [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Segment expenses	(792)	[1]		(771)	[2]	(1,297)	[3]	(1,159)	[4]
Travel and Entertainment [Member] | Research Segment [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Segment expenses	(628)	[1]		(661)	[2]	(1,017)	[3]	(960)	[4]
Travel and Entertainment [Member] | Consulting Segment Member | Operating Segments [Member]
Segment Reporting Information [Line Items]
Segment expenses	(130)	[1]		(79)	[2]	(242)	[3]	(163)	[4]
Travel and Entertainment [Member] | Events Segment [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Segment expenses	(34)	[1]		(31)	[2]	(38)	[3]	(36)	[4]
Software [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Segment expenses	(371)	[1]		(496)	[2]	(765)	[3]	(958)	[4]
Software [Member] | Research Segment [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Segment expenses	(354)	[1]		(492)	[2]	(731)	[3]	(950)	[4]
Software [Member] | Consulting Segment Member | Operating Segments [Member]
Segment Reporting Information [Line Items]
Segment expenses	0	[1]		0	[2]	0	[3]	0	[4]
Software [Member] | Events Segment [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Segment expenses	(17)	[1]		(4)	[2]	(34)	[3]	(8)	[4]
Other Segment Expenses [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Segment expenses	(38)	[1],[5]		(165)	[2],[6]	(107)	[3],[7]	(287)	[4],[8]
Other Segment Expenses [Member] | Research Segment [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Segment expenses	(27)	[1],[5]		(151)	[2],[6]	(71)	[3],[7]	(276)	[4],[8]
Other Segment Expenses [Member] | Consulting Segment Member | Operating Segments [Member]
Segment Reporting Information [Line Items]
Segment expenses	(4)	[1],[5]		(6)	[2],[6]	(14)	[3],[7]	(10)	[4],[8]
Other Segment Expenses [Member] | Events Segment [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Segment expenses	$ (7)	[1],[5]		$ (8)	[2],[6]	$ (22)	[3],[7]	$ (1)	[4],[8]

[1] The significant expense categories and amounts align with the segment-level information that is regularly provided to the chief operating decision maker.

[2] The significant expense categories and amounts align with the segment-level information that is regularly provided to the chief operating decision maker.

[3] The significant expense categories and amounts align with the segment-level information that is regularly provided to the chief operating decision maker.

[4] The significant expense categories and amounts align with the segment-level information that is regularly provided to the chief operating decision maker.

[5] Other segment expenses for each reportable segment includes office supplies, maintenance, and training expenses.

[6] Other segment expenses for each reportable segment includes office supplies, maintenance, and training expenses.

[7] Other segment expenses for each reportable segment includes office supplies, maintenance, and training expenses.

[8] Other segment expenses for each reportable segment includes office supplies, maintenance, and training expenses.