Goodwill and Intangible Assets (Detail) - Changes to Carrying Amount of Goodwill by Reporting Unit (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015
Goodwill [Roll Forward]
Balance, December 31, 2014	$ 586,665	[1]
Foreign currency translation adjustments and other	(6,755)
Balance, March 31, 2015	579,910
Research
Goodwill [Roll Forward]
Balance, December 31, 2014	445,460	[1]
Foreign currency translation adjustments and other	(5,816)
Balance, March 31, 2015	439,644
Consulting
Goodwill [Roll Forward]
Balance, December 31, 2014	99,417	[1]
Foreign currency translation adjustments and other	(835)
Balance, March 31, 2015	98,582
Events
Goodwill [Roll Forward]
Balance, December 31, 2014	41,788	[1]
Foreign currency translation adjustments and other	(104)
Balance, March 31, 2015	$ 41,684

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