Goodwill and Intangible Assets (Detail) - Changes to Carrying Amount of Goodwill by Reporting Unit $ in Thousands		6 Months Ended
		Jun. 30, 2015 USD ($)
Goodwill [Roll Forward]
Balance, December 31, 2014	[1]	$ 586,665
Foreign currency translation adjustments and other		(3,663)
Balance, June 30, 2015		583,002
Research
Goodwill [Roll Forward]
Balance, December 31, 2014	[1]	445,460
Foreign currency translation adjustments and other		(3,507)
Balance, June 30, 2015		441,953
Consulting
Goodwill [Roll Forward]
Balance, December 31, 2014	[1]	99,417
Foreign currency translation adjustments and other		(89)
Balance, June 30, 2015		99,328
Events
Goodwill [Roll Forward]
Balance, December 31, 2014	[1]	41,788
Foreign currency translation adjustments and other		(67)
Balance, June 30, 2015		$ 41,721

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