Operating Segment and Enterprise Wide Reporting - Schedule of Information about Reportable Segments (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Research services revenues									$ 216,471	$ 215,216	$ 210,268
Advisory services and events revenues									121,202	110,879	103,458
Total revenues	$ 90,377	$ 80,369	$ 89,733	$ 77,194	$ 83,446	$ 77,427	$ 87,821	$ 77,401	337,673	326,095	313,726
Segment expenses									119,494	112,165	110,184
Contribution margin (loss)									218,179	213,930	203,542
Selling, marketing, administrative and other expenses									(189,849)	(181,299)	(179,391)
Amortization of intangible assets									(781)	(831)	(891)
Reorganization costs										(1,026)	(4,433)
Other income and losses on investments									(178)	(65)	493
Income before income taxes									27,371	30,709	19,320
Product Segment [Member]
Segment Reporting Information [Line Items]
Research services revenues									216,471	215,216	210,268
Advisory services and events revenues									21,887	20,374	17,512
Total revenues									238,358	235,590	227,780
Segment expenses									45,205	41,528	38,615
Contribution margin (loss)									193,153	194,062	189,165
Research Segment [Member]
Segment Reporting Information [Line Items]
Advisory services and events revenues									45,054	44,631	42,422
Total revenues									45,054	44,631	42,422
Segment expenses									48,812	47,496	48,277
Contribution margin (loss)									(3,758)	(2,865)	(5,855)
Project Consulting [Member]
Segment Reporting Information [Line Items]
Advisory services and events revenues									54,261	45,874	43,524
Total revenues									54,261	45,874	43,524
Segment expenses									25,477	23,141	23,292
Contribution margin (loss)									$ 28,784	$ 22,733	$ 20,232