Operating Segments - Schedule of Information about Reportable Segments (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Total revenues	$ 77,749	$ 77,194
Segment expenses	29,694	27,224
Contribution margin (loss)	48,055	49,970
Selling, marketing, administrative and other expenses	(50,157)	(46,643)
Amortization of intangible assets	(186)	(191)
Other income (expense) and losses on investments	(143)	(194)
Income (loss) before income taxes	(2,431)	2,942
Research [Member]
Segment Reporting Information [Line Items]
Total revenues	34,643	35,524
Connect [Member]
Segment Reporting Information [Line Items]
Total revenues	12,564	11,637
Analytics [Member]
Segment Reporting Information [Line Items]
Total revenues	4,493	4,582
Research Services [Member]
Segment Reporting Information [Line Items]
Total revenues	51,700	51,743
Advisory Services [Member]
Segment Reporting Information [Line Items]
Total revenues	9,514	8,613
Consulting Services [Member]
Segment Reporting Information [Line Items]
Total revenues	14,571	14,337
Events [Member]
Segment Reporting Information [Line Items]
Total revenues		80
Analytics Services [Member]
Segment Reporting Information [Line Items]
Total revenues	1,964	2,421
Advisory Services and Events [Member]
Segment Reporting Information [Line Items]
Total revenues	26,049	25,451
Product Organization [Member]
Segment Reporting Information [Line Items]
Total revenues	53,664	54,244
Segment expenses	10,117	9,227
Contribution margin (loss)	43,547	45,017
Product Organization [Member] | Research [Member]
Segment Reporting Information [Line Items]
Total revenues	34,643	35,524
Product Organization [Member] | Connect [Member]
Segment Reporting Information [Line Items]
Total revenues	12,564	11,637
Product Organization [Member] | Analytics [Member]
Segment Reporting Information [Line Items]
Total revenues	4,493	4,582
Product Organization [Member] | Research Services [Member]
Segment Reporting Information [Line Items]
Total revenues	51,700	51,743
Product Organization [Member] | Events [Member]
Segment Reporting Information [Line Items]
Total revenues		80
Product Organization [Member] | Analytics Services [Member]
Segment Reporting Information [Line Items]
Total revenues	1,964	2,421
Product Organization [Member] | Advisory Services and Events [Member]
Segment Reporting Information [Line Items]
Total revenues	1,964	2,501
Research Organization [Member]
Segment Reporting Information [Line Items]
Total revenues	11,642	10,493
Segment expenses	12,713	12,143
Contribution margin (loss)	(1,071)	(1,650)
Research Organization [Member] | Advisory Services [Member]
Segment Reporting Information [Line Items]
Total revenues	9,480	8,536
Research Organization [Member] | Consulting Services [Member]
Segment Reporting Information [Line Items]
Total revenues	2,162	1,957
Research Organization [Member] | Advisory Services and Events [Member]
Segment Reporting Information [Line Items]
Total revenues	11,642	10,493
Project Consulting [Member]
Segment Reporting Information [Line Items]
Total revenues	12,443	12,457
Segment expenses	6,864	5,854
Contribution margin (loss)	5,579	6,603
Project Consulting [Member] | Advisory Services [Member]
Segment Reporting Information [Line Items]
Total revenues	34	77
Project Consulting [Member] | Consulting Services [Member]
Segment Reporting Information [Line Items]
Total revenues	12,409	12,380
Project Consulting [Member] | Advisory Services and Events [Member]
Segment Reporting Information [Line Items]
Total revenues	$ 12,443	$ 12,457