Operating Segments - Schedule of Information about Reportable Segments (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Total revenues	$ 108,596	$ 84,890	$ 337,428	$ 258,992
Segment expenses	40,571	30,909	134,863	94,739
Contribution margin	68,025	53,981	202,565	164,253
Selling, marketing, administrative and other expenses	(61,617)	(47,660)	(185,656)	(148,496)
Amortization of intangible assets	(5,654)	(402)	(16,963)	(770)
Acquisition and integration costs	(2,394)	(977)	(7,848)	(1,306)
Interest expense, other income and loss on investments	(1,794)	302	(6,631)	410
Income (loss) before income taxes	(3,434)	5,244	(14,533)	14,091
Research [Member]
Segment Reporting Information [Line Items]
Total revenues	55,023	38,199	161,108	113,897
Connect [Member]
Segment Reporting Information [Line Items]
Total revenues	14,256	12,890	41,894	37,992
Analytics [Member]
Segment Reporting Information [Line Items]
Total revenues	5,269	5,243	16,434	14,443
Research Services [Member]
Segment Reporting Information [Line Items]
Total revenues	74,548	56,332	219,436	166,332
Consulting [Member]
Segment Reporting Information [Line Items]
Total revenues	32,619	27,643	98,423	83,736
Events [Member]
Segment Reporting Information [Line Items]
Total revenues	1,429	915	19,569	8,924
Advisory Services and Events [Member]
Segment Reporting Information [Line Items]
Total revenues	34,048	28,558	117,992	92,660
Products Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	39,587	34,936	121,471	108,017
Segment expenses	18,374	17,800	59,982	54,841
Contribution margin	21,213	17,136	61,489	53,176
Products Segment [Member] | Connect [Member]
Segment Reporting Information [Line Items]
Total revenues	13,819	12,890	40,459	37,992
Products Segment [Member] | Analytics [Member]
Segment Reporting Information [Line Items]
Total revenues	5,269	5,243	16,434	14,443
Products Segment [Member] | Research Services [Member]
Segment Reporting Information [Line Items]
Total revenues	19,088	18,133	56,893	52,435
Products Segment [Member] | Consulting [Member]
Segment Reporting Information [Line Items]
Total revenues	19,298	15,888	56,156	46,658
Products Segment [Member] | Events [Member]
Segment Reporting Information [Line Items]
Total revenues	1,201	915	8,422	8,924
Products Segment [Member] | Advisory Services and Events [Member]
Segment Reporting Information [Line Items]
Total revenues	20,499	16,803	64,578	55,582
Research Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	52,217	49,954	156,431	150,975
Segment expenses	13,525	13,109	41,301	39,898
Contribution margin	38,692	36,845	115,130	111,077
Research Segment [Member] | Research [Member]
Segment Reporting Information [Line Items]
Total revenues	40,315	38,199	118,258	113,897
Research Segment [Member] | Research Services [Member]
Segment Reporting Information [Line Items]
Total revenues	40,315	38,199	118,258	113,897
Research Segment [Member] | Consulting [Member]
Segment Reporting Information [Line Items]
Total revenues	11,902	11,755	38,173	37,078
Research Segment [Member] | Advisory Services and Events [Member]
Segment Reporting Information [Line Items]
Total revenues	11,902	$ 11,755	38,173	$ 37,078
Sirius Decisions Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	16,792		59,526
Segment expenses	8,672		33,580
Contribution margin	8,120		25,946
Sirius Decisions Segment [Member] | Research [Member]
Segment Reporting Information [Line Items]
Total revenues	14,708		42,850
Sirius Decisions Segment [Member] | Connect [Member]
Segment Reporting Information [Line Items]
Total revenues	437		1,435
Sirius Decisions Segment [Member] | Research Services [Member]
Segment Reporting Information [Line Items]
Total revenues	15,145		44,285
Sirius Decisions Segment [Member] | Consulting [Member]
Segment Reporting Information [Line Items]
Total revenues	1,419		4,094
Sirius Decisions Segment [Member] | Events [Member]
Segment Reporting Information [Line Items]
Total revenues	228		11,147
Sirius Decisions Segment [Member] | Advisory Services and Events [Member]
Segment Reporting Information [Line Items]
Total revenues	$ 1,647		$ 15,241