Operating Segment and Enterprise Wide Reporting - Schedule of Information about Reportable Segments (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Segment Reporting Information [Line Items]
Research services revenues	$ 52,205	$ 50,622	$ 156,667	$ 153,737
Advisory services and events revenues	22,548	24,741	76,084	77,644
Total segment revenues	74,753	75,363	232,751	231,381
Segment expenses	27,440	28,900	86,810	88,634
Contribution margin (loss)	47,313	46,463	145,941	142,747
Selling, marketing, administrative and other expenses	(40,820)	(41,079)	(126,120)	(127,300)
Amortization of intangible assets	(224)	(530)	(669)	(1,605)
Reorganization (costs) credits	(928)	71	(4,433)	(1,817)
Other income and gains (losses) on investments	404	127	578	222
Income before income taxes	5,745	5,052	15,297	12,247
Products Segment [Member]
Segment Reporting Information [Line Items]
Research services revenues	52,205	50,622	156,667	153,737
Advisory services and events revenues	2,041	2,310	12,763	15,161
Total segment revenues	54,246	52,932	169,430	168,898
Segment expenses	7,918	8,298	27,799	28,543
Contribution margin (loss)	46,328	44,634	141,631	140,355
Research Segment [Member]
Segment Reporting Information [Line Items]
Advisory services and events revenues	9,804	10,925	31,454	37,573
Total segment revenues	9,804	10,925	31,454	37,573
Segment expenses	12,711	13,012	38,467	40,386
Contribution margin (loss)	(2,907)	(2,087)	(7,013)	(2,813)
Project Consulting [Member]
Segment Reporting Information [Line Items]
Advisory services and events revenues	10,703	11,506	31,867	24,910
Total segment revenues	10,703	11,506	31,867	24,910
Segment expenses	6,811	7,590	20,544	19,705
Contribution margin (loss)	$ 3,892	$ 3,916	$ 11,323	$ 5,205