Operating Segments - Schedule of Information about Reportable Segments (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Segment Reporting Information [Line Items]
Research services revenues	$ 55,023	$ 52,604	$ 108,271	$ 104,462
Advisory services and events revenues	32,798	30,207	56,951	53,536
Total revenues	87,821	82,811	165,222	157,998
Segment expenses	32,388	31,113	61,383	59,370
Contribution margin (loss)	55,433	51,698	103,839	98,628
Selling, marketing, administrative and other expenses	(43,740)	(41,300)	(88,210)	(85,300)
Amortization of intangible assets	(210)	(224)	(419)	(445)
Reorganization costs	(11)	(81)	(1,026)	(3,505)
Other income (expense) and gains (losses) on investments	419	(89)	91	174
Income before income taxes	11,891	10,004	14,275	9,552
Products Segment [Member]
Segment Reporting Information [Line Items]
Research services revenues	55,023	52,604	108,271	104,462
Advisory services and events revenues	9,157	8,255	11,858	10,722
Total revenues	64,180	60,859	120,129	115,184
Segment expenses	12,544	11,532	21,422	19,881
Contribution margin (loss)	51,636	49,327	98,707	95,303
Research Segment [Member]
Segment Reporting Information [Line Items]
Advisory services and events revenues	12,676	11,323	23,262	21,650
Total revenues	12,676	11,323	23,262	21,650
Segment expenses	12,822	12,808	26,159	25,756
Contribution margin (loss)	(146)	(1,485)	(2,897)	(4,106)
Project Consulting [Member]
Segment Reporting Information [Line Items]
Advisory services and events revenues	10,965	10,629	21,831	21,164
Total revenues	10,965	10,629	21,831	21,164
Segment expenses	7,022	6,773	13,802	13,733
Contribution margin (loss)	$ 3,943	$ 3,856	$ 8,029	$ 7,431