Condensed Consolidated Statements of Operations (Unaudited) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Revenues:
Research	$ 381,090us-gaap_SubscriptionRevenue	$ 348,114us-gaap_SubscriptionRevenue
Consulting	76,792us-gaap_ContractsRevenue	84,271us-gaap_ContractsRevenue
Events	13,304us-gaap_AdmissionsRevenue	14,317us-gaap_AdmissionsRevenue
Total revenues	471,186us-gaap_SalesRevenueNet	446,702us-gaap_SalesRevenueNet
Costs and expenses:
Cost of services and product development	177,762us-gaap_CostOfServices	170,821us-gaap_CostOfServices
Selling, general and administrative	229,522	204,617
Depreciation	7,989us-gaap_Depreciation	7,459us-gaap_Depreciation
Amortization of intangibles	2,139us-gaap_AmortizationOfIntangibleAssets	1,279us-gaap_AmortizationOfIntangibleAssets
Acquisition and integration charges	5,092	3,356
Total costs and expenses	422,504us-gaap_CostsAndExpenses	387,532us-gaap_CostsAndExpenses
Operating income	48,682us-gaap_OperatingIncomeLoss	59,170us-gaap_OperatingIncomeLoss
Interest expense, net	(3,480)us-gaap_InterestExpense	(2,250)us-gaap_InterestExpense
Other expense, net	(962)us-gaap_OtherNonoperatingIncomeExpense	(229)us-gaap_OtherNonoperatingIncomeExpense
Income before income taxes	44,240	56,691
Provision for income taxes	15,889us-gaap_IncomeTaxExpenseBenefit	18,955us-gaap_IncomeTaxExpenseBenefit
Net income	$ 28,351us-gaap_NetIncomeLoss	$ 37,736us-gaap_NetIncomeLoss
Earnings per common share:
Basic (in Dollars per share)	$ 0.33us-gaap_EarningsPerShareBasic	$ 0.41us-gaap_EarningsPerShareBasic
Diluted (in Dollars per share)	$ 0.32us-gaap_EarningsPerShareDiluted	$ 0.40us-gaap_EarningsPerShareDiluted
Weighted average shares outstanding:
Basic (in shares)	86,539	91,669
Diluted (in shares)	87,773	93,209

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