Operating Segments - Schedule of Information about Reportable Segments (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Total revenues	$ 84,890	$ 80,369	$ 258,992	$ 247,296
Segment expenses	31,221	28,160	95,646	88,184
Contribution margin (loss)	53,669	52,209	163,346	159,112
Selling, marketing, administrative and other expenses	(47,348)	(45,263)	(147,589)	(138,432)
Amortization of intangible assets	(402)	(197)	(770)	(582)
Acquisition and integration cost	(977)		(1,306)
Other income and losses on investments	302	(626)	410	(749)
Income before income taxes	5,244	6,123	14,091	19,349
Research [Member]
Segment Reporting Information [Line Items]
Total revenues	38,309	36,894	114,007	110,550
Connect [Member]
Segment Reporting Information [Line Items]
Total revenues	12,780	12,506	37,882	36,103
Analytics [Member]
Segment Reporting Information [Line Items]
Total revenues	5,243	4,835	14,443	13,900
Research Services [Member]
Segment Reporting Information [Line Items]
Total revenues	56,332	54,235	166,332	160,553
Advisory Services [Member]
Segment Reporting Information [Line Items]
Total revenues	9,325	8,378	29,954	26,143
Consulting Services [Member]
Segment Reporting Information [Line Items]
Total revenues	18,318	16,653	53,782	52,381
Events [Member]
Segment Reporting Information [Line Items]
Total revenues	915	1,103	8,924	8,219
Advisory Services and Events [Member]
Segment Reporting Information [Line Items]
Total revenues	28,558	26,134	92,660	86,743
Product Organization [Member]
Segment Reporting Information [Line Items]
Total revenues	59,277	57,588	181,335	176,267
Segment expenses	11,521	9,764	36,534	32,788
Contribution margin (loss)	47,756	47,824	144,801	143,479
Product Organization [Member] | Research [Member]
Segment Reporting Information [Line Items]
Total revenues	38,309	36,894	114,007	110,550
Product Organization [Member] | Connect [Member]
Segment Reporting Information [Line Items]
Total revenues	12,780	12,506	37,882	36,103
Product Organization [Member] | Analytics [Member]
Segment Reporting Information [Line Items]
Total revenues	5,243	4,835	14,443	13,900
Product Organization [Member] | Research Services [Member]
Segment Reporting Information [Line Items]
Total revenues	56,332	54,235	166,332	160,553
Product Organization [Member] | Consulting Services [Member]
Segment Reporting Information [Line Items]
Total revenues	2,030	2,250	6,079	7,495
Product Organization [Member] | Events [Member]
Segment Reporting Information [Line Items]
Total revenues	915	1,103	8,924	8,219
Product Organization [Member] | Advisory Services and Events [Member]
Segment Reporting Information [Line Items]
Total revenues	2,945	3,353	15,003	15,714
Research Organization [Member]
Segment Reporting Information [Line Items]
Total revenues	11,688	10,379	36,970	32,279
Segment expenses	12,688	11,953	38,182	36,510
Contribution margin (loss)	(1,000)	(1,574)	(1,212)	(4,231)
Research Organization [Member] | Advisory Services [Member]
Segment Reporting Information [Line Items]
Total revenues	9,171	8,297	29,664	25,889
Research Organization [Member] | Consulting Services [Member]
Segment Reporting Information [Line Items]
Total revenues	2,517	2,082	7,306	6,390
Research Organization [Member] | Advisory Services and Events [Member]
Segment Reporting Information [Line Items]
Total revenues	11,688	10,379	36,970	32,279
Project Consulting [Member]
Segment Reporting Information [Line Items]
Total revenues	13,925	12,402	40,687	38,750
Segment expenses	7,012	6,443	20,930	18,886
Contribution margin (loss)	6,913	5,959	19,757	19,864
Project Consulting [Member] | Advisory Services [Member]
Segment Reporting Information [Line Items]
Total revenues	154	81	290	254
Project Consulting [Member] | Consulting Services [Member]
Segment Reporting Information [Line Items]
Total revenues	13,771	12,321	40,397	38,496
Project Consulting [Member] | Advisory Services and Events [Member]
Segment Reporting Information [Line Items]
Total revenues	$ 13,925	$ 12,402	$ 40,687	$ 38,750