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, 2014
Goodwill [Roll Forward]
Balance, December 31, 2013	$ 519,203	[1]
Addition due to acquisition	78,592	[2]
Foreign currency translation adjustments and other	(4,583)
Balance, September 30, 2014	593,212
Research
Goodwill [Roll Forward]
Balance, December 31, 2013	376,568	[1]
Foreign currency translation adjustments and other	(4,111)
Balance, September 30, 2014	451,049
Consulting
Goodwill [Roll Forward]
Balance, December 31, 2013	100,677	[1]
Addition due to acquisition	0	[2]
Foreign currency translation adjustments and other	(383)
Balance, September 30, 2014	100,294
Events
Goodwill [Roll Forward]
Balance, December 31, 2013	41,958	[1]
Addition due to acquisition	0	[2]
Foreign currency translation adjustments and other	(89)
Balance, September 30, 2014	$ 41,869

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

[2] The Company made three acquisitions during the nine months ended September 30, 2014 (See Note 1 for additional discussion). All of the recorded goodwill from these acquisitions has been included in the Research segment.