Business and Significant Accounting Policies (Detail) - Changes to the Carrying Amount of Goodwill by Reporting Unit - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Goodwill [Roll Forward]
Beginning Balance	$ 586,665	$ 519,203
Addition due to acquisition	138,053	78,373
Foreign currency translation adjustments	(9,359)	(10,911)
Ending Balance	715,359	586,665
Research
Goodwill [Roll Forward]
Beginning Balance	445,460	376,568
Addition due to acquisition	138,053	78,373
Foreign currency translation adjustments	(8,221)	(9,481)
Ending Balance	575,292	445,460
Consulting
Goodwill [Roll Forward]
Beginning Balance	99,417	100,677
Addition due to acquisition	0	0
Foreign currency translation adjustments	(1,005)	(1,260)
Ending Balance	98,412	99,417
Events
Goodwill [Roll Forward]
Beginning Balance	41,788	41,958
Addition due to acquisition	0	0
Foreign currency translation adjustments	(133)	(170)
Ending Balance	$ 41,655	$ 41,788