Operating Segment and Enterprise Wide Reporting - Summary of Information about Reportable Segments (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Research services revenues									$ 207,517	$ 202,843	$ 203,091
Advisory services and events revenues									104,545	94,807	89,940
Total segment revenues	80,681	75,363	82,947	73,071	77,521	69,815	78,953	71,361	312,062	297,650	293,031
Segment expenses									120,009	110,769	107,017
Contribution margin (loss)									192,053	186,881	186,014
Selling, marketing, administrative and other expenses									(169,852)	(160,913)	(151,388)
Amortization of intangible assets									(2,171)	(2,230)	(2,445)
Reorganization costs									(1,817)	(1,905)	(1,421)
Other income and gains (losses) on investments									176	(1,841)	1,394
Income before income taxes									18,389	19,992	32,154
Products Segment [Member]
Segment Reporting Information [Line Items]
Research services revenues									207,517	202,843	203,091
Advisory services and events revenues									20,759	19,376	17,905
Total segment revenues									228,276	222,219	220,996
Segment expenses									39,466	36,384	36,468
Contribution margin (loss)									188,810	185,835	184,528
Research Segment [Member]
Segment Reporting Information [Line Items]
Advisory services and events revenues									48,658	57,865	56,928
Total segment revenues									48,658	57,865	56,928
Segment expenses									53,307	58,685	57,440
Contribution margin (loss)									(4,649)	(820)	(512)
Project Consulting [Member]
Segment Reporting Information [Line Items]
Advisory services and events revenues									35,128	17,566	15,107
Total segment revenues									35,128	17,566	15,107
Segment expenses									27,236	15,700	13,109
Contribution margin (loss)									$ 7,892	$ 1,866	$ 1,998