Interim Consolidated Financial Statements - Summary of Effect of Adopting ASC 606 In Consolidated Statement of Income (Loss) (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenues:
Total revenues	$ 77,749	$ 77,194
Operating expenses:
Cost of services and fulfillment	34,105	31,396
Selling and marketing	33,011	30,622
Total operating expenses	80,037	74,058
Income from operations	(2,288)	3,136
Income before income taxes	(2,431)	2,942
Income tax provision	(698)	(88)
Net loss	$ (1,733)	$ 3,030
Basic loss per common share	$ (0.10)	$ 0.17
Diluted loss per common share	$ (0.10)	$ 0.16
Research Services [Member]
Revenues:
Total revenues	$ 51,700	$ 51,743
Advisory Services and Events [Member]
Revenues:
Total revenues	26,049	$ 25,451
ASC 606 [Member] | Amounts as if Previous Guidance in Effect [Member]
Revenues:
Total revenues	80,005
Operating expenses:
Cost of services and fulfillment	34,090
Selling and marketing	33,073
Total operating expenses	80,084
Income from operations	(79)
Income before income taxes	(222)
Income tax provision	(13)
Net loss	$ (209)
Basic loss per common share	$ (0.01)
Diluted loss per common share	$ (0.01)
ASC 606 [Member] | Research Services [Member] | Amounts as if Previous Guidance in Effect [Member]
Revenues:
Total revenues	$ 53,387
ASC 606 [Member] | Advisory Services and Events [Member] | Amounts as if Previous Guidance in Effect [Member]
Revenues:
Total revenues	$ 26,618