Operating Segments - Schedule of Information about Reportable Segments (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information [Line Items]
Total revenues	$ 128,183	$ 96,353	$ 228,832	$ 174,102
Segment expenses	52,676	34,363	94,292	63,830
Contribution margin	75,507	61,990	134,540	110,272
Selling, marketing, administrative and other expenses	(63,299)	(50,452)	(124,039)	(100,836)
Amortization of intangible assets	(5,099)	(182)	(11,309)	(368)
Acquisition and integration costs	(2,487)	(329)	(5,454)	(329)
Interest, other expense and loss on investments	(2,179)	251	(4,837)	108
Income (loss) before income taxes	2,443	11,278	(11,099)	8,847
Research [Member]
Segment Reporting Information [Line Items]
Total revenues	56,305	41,055	106,085	75,698
Connect [Member]
Segment Reporting Information [Line Items]
Total revenues	14,067	12,538	27,638	25,102
Analytics [Member]
Segment Reporting Information [Line Items]
Total revenues	5,907	4,707	11,165	9,200
Research Services [Member]
Segment Reporting Information [Line Items]
Total revenues	76,279	58,300	144,888	110,000
Consulting [Member]
Segment Reporting Information [Line Items]
Total revenues	34,017	30,044	65,804	56,093
Events [Member]
Segment Reporting Information [Line Items]
Total revenues	17,887	8,009	18,140	8,009
Advisory Services and Events [Member]
Segment Reporting Information [Line Items]
Total revenues	51,904	38,053	83,944	64,102
Products Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	45,647	41,617	81,884	73,081
Segment expenses	23,431	20,985	41,608	37,041
Contribution margin	22,216	20,632	40,276	36,040
Products Segment [Member] \| Connect [Member]
Segment Reporting Information [Line Items]
Total revenues	13,525	12,538	26,640	25,102
Products Segment [Member] \| Analytics [Member]
Segment Reporting Information [Line Items]
Total revenues	5,907	4,707	11,165	9,200
Products Segment [Member] \| Research Services [Member]
Segment Reporting Information [Line Items]
Total revenues	19,432	17,245	37,805	34,302
Products Segment [Member] \| Consulting [Member]
Segment Reporting Information [Line Items]
Total revenues	19,021	16,363	36,858	30,770
Products Segment [Member] \| Events [Member]
Segment Reporting Information [Line Items]
Total revenues	7,194	8,009	7,221	8,009
Products Segment [Member] \| Advisory Services and Events [Member]
Segment Reporting Information [Line Items]
Total revenues	26,215	24,372	44,079	38,779
Research Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	55,009	54,736	104,214	101,021
Segment expenses	13,747	13,378	27,776	26,789
Contribution margin	41,262	41,358	76,438	74,232
Research Segment [Member] \| Research [Member]
Segment Reporting Information [Line Items]
Total revenues	41,506	41,055	77,943	75,698
Research Segment [Member] \| Research Services [Member]
Segment Reporting Information [Line Items]
Total revenues	41,506	41,055	77,943	75,698
Research Segment [Member] \| Consulting [Member]
Segment Reporting Information [Line Items]
Total revenues	13,503	13,681	26,271	25,323
Research Segment [Member] \| Advisory Services and Events [Member]
Segment Reporting Information [Line Items]
Total revenues	13,503	$ 13,681	26,271	$ 25,323
Sirius Decisions Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	27,527		42,734
Segment expenses	15,498		24,908
Contribution margin	12,029		17,826
Sirius Decisions Segment [Member] \| Research [Member]
Segment Reporting Information [Line Items]
Total revenues	14,799		28,142
Sirius Decisions Segment [Member] \| Connect [Member]
Segment Reporting Information [Line Items]
Total revenues	542		998
Sirius Decisions Segment [Member] \| Research Services [Member]
Segment Reporting Information [Line Items]
Total revenues	15,341		29,140
Sirius Decisions Segment [Member] \| Consulting [Member]
Segment Reporting Information [Line Items]
Total revenues	1,493		2,675
Sirius Decisions Segment [Member] \| Events [Member]
Segment Reporting Information [Line Items]
Total revenues	10,693		10,919
Sirius Decisions Segment [Member] \| Advisory Services and Events [Member]
Segment Reporting Information [Line Items]
Total revenues	$ 12,186		$ 13,594

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