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, 2014		Dec. 31, 2013
Goodwill [Roll Forward]
Beginning Balance	$ 519,203		$ 519,506	[1]
Addition due to acquisition	78,373	[2]
Foreign currency translation adjustments	(10,911)		(303)
Ending Balance	586,665		519,203
Research
Goodwill [Roll Forward]
Beginning Balance	376,568		377,225	[1]
Foreign currency translation adjustments	(9,481)		(657)
Ending Balance	445,460		376,568
Consulting
Goodwill [Roll Forward]
Beginning Balance	100,677		100,349	[1]
Addition due to acquisition	0	[2]
Foreign currency translation adjustments	(1,260)		328
Ending Balance	99,417		100,677
Events
Goodwill [Roll Forward]
Beginning Balance	41,958		41,932	[1]
Addition due to acquisition	0	[2]
Foreign currency translation adjustments	(170)		26
Ending Balance	$ 41,788		$ 41,958

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

[2] The addition is due to the 2014 Acquisitions (See Note 2—Acquisitions for additional discussion). All of the recorded goodwill from these acquisitions has been included in the Research segment.