Goodwill and Intangible Assets (Detail) - Changes to Carrying Amount of Goodwill by Reporting Unit (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2013
Goodwill [Roll Forward]
Balance, December 31, 2012	$ 519,506	[1]
Foreign currency translation adjustments and other	(951)	[2]
Balance, September 30, 2013	518,555
Research
Goodwill [Roll Forward]
Balance, December 31, 2012	377,225	[1]
Foreign currency translation adjustments and other	(901)	[2]
Balance, September 30, 2013	376,324
Consulting
Goodwill [Roll Forward]
Balance, December 31, 2012	100,349	[1]
Foreign currency translation adjustments and other	(59)	[2]
Balance, September 30, 2013	100,290
Events
Goodwill [Roll Forward]
Balance, December 31, 2012	41,932	[1]
Foreign currency translation adjustments and other	9	[2]
Balance, September 30, 2013	$ 41,941

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

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