Business and Significant Accounting Policies (Detail) - Changes to the Carrying Amount of Goodwill by Reporting Unit (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011	Dec. 31, 2010
Balance	$ 519,506		$ 508,550	$ 510,265	[1]
Addition due to acquisition (2)	7,455	[2]
Foreign currency translation adjustments	3,501		(1,715)
Research [Member]
Balance	377,225		366,980	368,521	[1]
Addition due to acquisition (2)	7,455	[2]
Foreign currency translation adjustments	2,790		(1,541)
Consulting [Member]
Balance	100,349		99,677	99,817	[1]
Addition due to acquisition (2)		[2]
Foreign currency translation adjustments	672		(140)
Events [Member]
Balance	41,932		41,893	41,927	[1]
Addition due to acquisition (2)		[2]
Foreign currency translation adjustments	$ 39		$ (34)

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

[2] The Company acquired Ideas International in mid-2012 and recorded $7.5 million of goodwill. All of the recorded goodwill resulting from the acquisition has been included in the Research segment. See Note 2-Acquisitions above for additional information.