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, 2014
Goodwill [Roll Forward]
Balance, December 31, 2013	$ 519,203	[1]
Addition due to acquisition	78,675	[2]
Foreign currency translation adjustments and other	853
Balance, June 30, 2014	598,731
Research
Goodwill [Roll Forward]
Balance, December 31, 2013	376,568	[1]
Addition due to acquisition	78,675	[2]
Foreign currency translation adjustments and other	523
Balance, June 30, 2014	455,766
Consulting
Goodwill [Roll Forward]
Balance, December 31, 2013	100,677	[1]
Addition due to acquisition	0	[2]
Foreign currency translation adjustments and other	345
Balance, June 30, 2014	101,022
Events
Goodwill [Roll Forward]
Balance, December 31, 2013	41,958	[1]
Addition due to acquisition	0	[2]
Foreign currency translation adjustments and other	(15)
Balance, June 30, 2014	$ 41,943

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

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