Goodwill and Intangible Assets (Detail) - Changes to Carrying Amount of Goodwill by Reporting Unit (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013
Goodwill [Roll Forward]
Balance, December 31, 2012	$ 519,506	[1]
Foreign currency translation adjustments and other	(3,362)	[2]
Balance, June 30, 2013	516,144
Research
Goodwill [Roll Forward]
Balance, December 31, 2012	377,225	[1]
Foreign currency translation adjustments and other	(2,814)	[2]
Balance, June 30, 2013	374,411
Consulting
Goodwill [Roll Forward]
Balance, December 31, 2012	100,349	[1]
Foreign currency translation adjustments and other	(529)	[2]
Balance, June 30, 2013	99,820
Events
Goodwill [Roll Forward]
Balance, December 31, 2012	41,932	[1]
Foreign currency translation adjustments and other	(19)	[2]
Balance, June 30, 2013	$ 41,913

[1] The Company does not have any accumulated goodwill impairment losses.

[2] Includes the impact of foreign currency translation and certain immaterial goodwill adjustments.