Business and Significant Accounting Policies (Detail) - Changes to the Carrying Amount of Goodwill by Reporting Unit - USD ($)	3 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Goodwill [Line Items]
Goodwill, impairment loss	$ 0
Goodwill [Roll Forward]
Beginning Balance		$ 715,359,000	$ 586,665,000
Additions due to acquisition		34,308,000	138,053,000
Foreign currency translation adjustments		(11,214,000)	(9,359,000)
Ending Balance		738,453,000	715,359,000
Research
Goodwill [Roll Forward]
Beginning Balance		575,292,000	445,460,000
Additions due to acquisition		28,465,000	138,053,000
Foreign currency translation adjustments		(8,307,000)	(8,221,000)
Ending Balance		595,450,000	575,292,000
Consulting
Goodwill [Roll Forward]
Beginning Balance		98,412,000	99,417,000
Additions due to acquisition		0	0
Foreign currency translation adjustments		(1,932,000)	(1,005,000)
Ending Balance		96,480,000	98,412,000
Events
Goodwill [Roll Forward]
Beginning Balance		41,655,000	41,788,000
Additions due to acquisition		5,843,000	0
Foreign currency translation adjustments		(975,000)	(133,000)
Ending Balance		$ 46,523,000	$ 41,655,000