Goodwill and Intangible Assets (Detail) - Changes to Carrying Amount of Goodwill by Reporting Unit - USD ($)	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Goodwill [Line Items]
Goodwill, Impaired, Accumulated Impairment Loss	$ 0
Goodwill impairment		$ 0
Goodwill [Roll Forward]
Balance, December 31, 2015	715,359,000
Additions due to acquisitions	31,702,000
Foreign currency translation adjustments	(4,135,000)
Balance, September 30, 2016	742,926,000
Research
Goodwill [Roll Forward]
Balance, December 31, 2015	575,292,000
Additions due to acquisitions	25,744,000
Foreign currency translation adjustments	(2,291,000)
Balance, September 30, 2016	598,745,000
Consulting
Goodwill [Roll Forward]
Balance, December 31, 2015	98,412,000
Additions due to acquisitions	0
Foreign currency translation adjustments	(1,188,000)
Balance, September 30, 2016	97,224,000
Events
Goodwill [Roll Forward]
Balance, December 31, 2015	41,655,000
Additions due to acquisitions	5,958,000
Foreign currency translation adjustments	(656,000)
Balance, September 30, 2016	$ 46,957,000