Interim Consolidated Financial Statements - Summary of Effect of Adopting ASC 606 In Consolidated Statement of Income (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenues:
Total revenues	$ 84,890	$ 80,369	$ 258,992	$ 247,296
Operating expenses:
Selling and marketing	31,051	29,225	96,771	90,355
Total operating expenses	79,948	73,620	245,311	227,198
Income from operations	4,942	6,749	13,681	20,098
Income before income taxes	5,244	6,123	14,091	19,349
Income tax provision	1,294	2,170	4,086	6,302
Net income	$ 3,950	$ 3,953	$ 10,005	$ 13,047
Basic income per common share	$ 0.22	$ 0.22	$ 0.55	$ 0.73
Diluted income per common share	$ 0.21	$ 0.22	$ 0.55	$ 0.72
Research Services [Member]
Revenues:
Total revenues	$ 56,332	$ 54,235	$ 166,332	$ 160,553
Advisory Services and Events [Member]
Revenues:
Total revenues	28,558	26,134	92,660	86,743
Cost of Services and Fulfillment [Member]
Operating expenses:
Cost of services and fulfillment	34,361	$ 32,508	107,537	$ 100,814
ASC 606 [Member] | Amounts as if Previous Guidance in Effect [Member]
Revenues:
Total revenues	84,006		260,480
Operating expenses:
Selling and marketing	30,871		96,502
Total operating expenses	80,245		245,346
Income from operations	3,761		15,134
Income before income taxes	4,063		15,544
Income tax provision	927		4,564
Net income	$ 3,136		$ 10,980
Basic income per common share	$ 0.17		$ 0.61
Diluted income per common share	$ 0.17		$ 0.60
ASC 606 [Member] | Research Services [Member] | Amounts as if Previous Guidance in Effect [Member]
Revenues:
Total revenues	$ 55,719		$ 167,429
ASC 606 [Member] | Advisory Services and Events [Member] | Amounts as if Previous Guidance in Effect [Member]
Revenues:
Total revenues	28,287		93,051
ASC 606 [Member] | Cost of Services and Fulfillment [Member] | Amounts as if Previous Guidance in Effect [Member]
Operating expenses:
Cost of services and fulfillment	$ 34,838		$ 107,841