Operating Segments - Schedule of Information about Reportable Segments (Detail) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Segment Reporting Information [Line Items]
Total revenues	$ 100,649	$ 77,749
Segment expenses	41,616	29,467
Contribution margin	59,033	48,282
Selling, marketing, administrative and other expenses	(60,740)	(50,384)
Amortization of intangible assets	(6,210)	(186)
Acquisition and integration costs	(2,967)
Interest and other expense and loss on investments	(2,658)	(143)
Loss before income taxes	(13,542)	(2,431)
Research [Member]
Segment Reporting Information [Line Items]
Total revenues	49,780	34,643
Connect [Member]
Segment Reporting Information [Line Items]
Total revenues	13,571	12,564
Analytics [Member]
Segment Reporting Information [Line Items]
Total revenues	5,258	4,493
Research Services [Member]
Segment Reporting Information [Line Items]
Total revenues	68,609	51,700
Consulting [Member]
Segment Reporting Information [Line Items]
Total revenues	31,787	26,049
Events [Member]
Segment Reporting Information [Line Items]
Total revenues	253
Advisory Services and Events [Member]
Segment Reporting Information [Line Items]
Total revenues	32,040	26,049
Products Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	36,237	31,464
Segment expenses	18,177	16,056
Contribution margin	18,060	15,408
Products Segment [Member] \| Connect [Member]
Segment Reporting Information [Line Items]
Total revenues	13,115	12,564
Products Segment [Member] \| Analytics [Member]
Segment Reporting Information [Line Items]
Total revenues	5,258	4,493
Products Segment [Member] \| Research Services [Member]
Segment Reporting Information [Line Items]
Total revenues	18,373	17,057
Products Segment [Member] \| Consulting [Member]
Segment Reporting Information [Line Items]
Total revenues	17,837	14,407
Products Segment [Member] \| Events [Member]
Segment Reporting Information [Line Items]
Total revenues	27
Products Segment [Member] \| Advisory Services and Events [Member]
Segment Reporting Information [Line Items]
Total revenues	17,864	14,407
Research Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	49,205	46,285
Segment expenses	14,029	13,411
Contribution margin	35,176	32,874
Research Segment [Member] \| Research [Member]
Segment Reporting Information [Line Items]
Total revenues	36,437	34,643
Research Segment [Member] \| Research Services [Member]
Segment Reporting Information [Line Items]
Total revenues	36,437	34,643
Research Segment [Member] \| Consulting [Member]
Segment Reporting Information [Line Items]
Total revenues	12,768	11,642
Research Segment [Member] \| Advisory Services and Events [Member]
Segment Reporting Information [Line Items]
Total revenues	12,768	$ 11,642
SiriusDecisions Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	15,207
Segment expenses	9,410
Contribution margin	5,797
SiriusDecisions Segment [Member] \| Research [Member]
Segment Reporting Information [Line Items]
Total revenues	13,343
SiriusDecisions Segment [Member] \| Connect [Member]
Segment Reporting Information [Line Items]
Total revenues	456
SiriusDecisions Segment [Member] \| Research Services [Member]
Segment Reporting Information [Line Items]
Total revenues	13,799
SiriusDecisions Segment [Member] \| Consulting [Member]
Segment Reporting Information [Line Items]
Total revenues	1,182
SiriusDecisions Segment [Member] \| Events [Member]
Segment Reporting Information [Line Items]
Total revenues	226
SiriusDecisions Segment [Member] \| Advisory Services and Events [Member]
Segment Reporting Information [Line Items]
Total revenues	$ 1,408

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