Goodwill and Intangible Assets (Detail) - Changes to Carrying Amount of Goodwill by Reporting Unit - USD ($)	3 Months Ended
Mar. 31, 2017	Sep. 30, 2016
Goodwill [Line Items]
Goodwill, accumulated impairment losses	$ 0
Goodwill impairment		$ 0
Goodwill [Roll Forward]
Balance, December 31, 2016	738,453,000
Additions due to acquisitions	114,222,000
Foreign currency translation adjustments	2,655,000
Balance, March 31, 2017	855,330,000
Research
Goodwill [Roll Forward]
Balance, December 31, 2016	595,450,000
Additions due to acquisitions	114,222,000
Foreign currency translation adjustments	2,376,000
Balance, March 31, 2017	712,048,000
Consulting
Goodwill [Roll Forward]
Balance, December 31, 2016	96,480,000
Additions due to acquisitions	0
Foreign currency translation adjustments	197,000
Balance, March 31, 2017	96,677,000
Events
Goodwill [Roll Forward]
Balance, December 31, 2016	46,523,000
Additions due to acquisitions	0
Foreign currency translation adjustments	82,000
Balance, March 31, 2017	$ 46,605,000

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

Reference 2: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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