Changes to the Carrying Amount of Goodwill by Reporting Segments (Detail) (USD $) In Thousands	6 Months Ended
	Jun. 30, 2011
Goodwill [Line Items]
Beginning Balance	$ 510,265
Foreign currency translation adjustments	5,762
Ending Balance	516,027	[1]
Research
Goodwill [Line Items]
Beginning Balance	368,521
Foreign currency translation adjustments	4,862
Ending Balance	373,383	[1]
Consulting
Goodwill [Line Items]
Beginning Balance	99,817
Foreign currency translation adjustments	798
Ending Balance	100,615	[1]
Events
Goodwill [Line Items]
Beginning Balance	41,927
Foreign currency translation adjustments	102
Ending Balance	$ 42,029	[1]

[1] The Company did not record any goodwill impairment losses during the six months ended June 30, 2011. In addition, the Company does not have any accumulated goodwill impairment losses.