Operating Segment and Enterprise Wide Reporting - Summary of Information about Reportable Segments (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Segment Reporting Information [Line Items]
Research services revenues	$ 52,604	$ 52,322	$ 104,462	$ 103,115
Advisory services and events revenues	30,207	30,625	53,536	52,903
Total segment revenues	82,811	82,947	157,998	156,018
Segment expenses	31,113	31,731	59,370	59,734
Contribution margin (loss)	51,698	51,216	98,628	96,284
Selling, marketing, administrative and other expenses	(41,300)	(42,561)	(85,300)	(86,221)
Amortization of intangible assets	(224)	(536)	(445)	(1,075)
Reorganization costs	(81)	(1,039)	(3,505)	(1,888)
Other income and gains (losses) on investments	(89)	122	174	95
Income before income taxes	10,004	7,202	9,552	7,195
Products Segment [Member]
Segment Reporting Information [Line Items]
Research services revenues	52,604	52,322	104,462	103,115
Advisory services and events revenues	8,255	9,994	10,722	12,851
Total segment revenues	60,859	62,316	115,184	115,966
Segment expenses	11,532	11,894	19,881	20,245
Contribution margin (loss)	49,327	50,422	95,303	95,721
Research Segment [Member]
Segment Reporting Information [Line Items]
Advisory services and events revenues	11,323	12,672	21,650	26,648
Total segment revenues	11,323	12,672	21,650	26,648
Segment expenses	12,808	13,399	25,756	27,374
Contribution margin (loss)	(1,485)	(727)	(4,106)	(726)
Project Consulting [Member]
Segment Reporting Information [Line Items]
Advisory services and events revenues	10,629	7,959	21,164	13,404
Total segment revenues	10,629	7,959	21,164	13,404
Segment expenses	6,773	6,438	13,733	12,115
Contribution margin (loss)	$ 3,856	$ 1,521	$ 7,431	$ 1,289