Operating Segment and Enterprise Wide Reporting - Schedule of Information about Reportable Segments (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Research services revenues									$ 210,268	$ 207,517	$ 202,843
Advisory services and events revenues									103,458	104,545	94,807
Total segment revenues	$ 80,975	$ 74,753	$ 82,811	$ 75,187	$ 80,681	$ 75,363	$ 82,947	$ 73,071	313,726	312,062	297,650
Segment expenses									117,445	120,009	110,769
Contribution margin (loss)									196,281	192,053	186,881
Selling, marketing, administrative and other expenses									(172,130)	(169,852)	(160,913)
Amortization of intangible assets									(891)	(2,171)	(2,230)
Reorganization costs									(4,433)	(1,817)	(1,905)
Other income and losses on investments									493	176	(1,841)
Income before income taxes									19,320	18,389	19,992
Products Segment [Member]
Segment Reporting Information [Line Items]
Research services revenues									210,268	207,517	202,843
Advisory services and events revenues									17,512	20,759	19,376
Total segment revenues									227,780	228,276	222,219
Segment expenses									38,615	39,466	36,384
Contribution margin (loss)									189,165	188,810	185,835
Research Segment [Member]
Segment Reporting Information [Line Items]
Advisory services and events revenues									42,997	48,658	57,865
Total segment revenues									42,997	48,658	57,865
Segment expenses									51,510	53,307	58,685
Contribution margin (loss)									(8,513)	(4,649)	(820)
Project Consulting [Member]
Segment Reporting Information [Line Items]
Advisory services and events revenues									42,949	35,128	17,566
Total segment revenues									42,949	35,128	17,566
Segment expenses									27,320	27,236	15,700
Contribution margin (loss)									$ 15,629	$ 7,892	$ 1,866