Goodwill and Intangible Assets (Detail) - Changes to Carrying Amount of Goodwill by Reporting Unit - USD ($)	3 Months Ended	9 Months Ended
	Mar. 31, 2016	Sep. 30, 2015
Goodwill [Line Items]
Goodwill impairment		$ 0
Goodwill [Roll Forward]
Balance, December 31, 2015	$ 715,359,000
Additions due to acquisitions	1,913,000
Foreign currency translation adjustments and other	762,000
Balance, March 31, 2016	718,034,000
Research
Goodwill [Roll Forward]
Balance, December 31, 2015	575,292,000
Additions due to acquisitions	1,913,000
Foreign currency translation adjustments and other	1,161,000
Balance, March 31, 2016	578,366,000
Consulting
Goodwill [Roll Forward]
Balance, December 31, 2015	98,412,000
Additions due to acquisitions	0
Foreign currency translation adjustments and other	(427,000)
Balance, March 31, 2016	97,985,000
Events
Goodwill [Roll Forward]
Balance, December 31, 2015	41,655,000
Additions due to acquisitions	0
Foreign currency translation adjustments and other	28,000
Balance, March 31, 2016	$ 41,683,000