Condensed Consolidated Statements of Operations (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Revenues:
Research	$ 316,518	$ 284,048	$ 938,082	$ 836,970
Consulting	70,149	71,731	228,710	222,970
Events	24,038	18,627	96,714	81,119
Total revenues	410,705	374,406	1,263,506	1,141,059
Costs and expenses:
Cost of services and product development	161,735	151,143	503,376	458,853
Selling, general and administrative	181,546	164,888	547,653	492,627
Depreciation	7,258	6,301	21,375	18,378
Amortization of intangibles	1,351	1,362	4,089	3,029
Acquisition and integration charges	72	944	278	2,126
Total costs and expenses	351,962	324,638	1,076,771	975,013
Operating income	58,743	49,768	186,735	166,046
Interest expense, net	(2,124)	(2,209)	(6,704)	(6,557)
Other expense, net	5	(748)	(64)	(1,802)
Income before income taxes	56,624	46,811	179,967	157,687
Provision for income taxes	18,430	15,436	58,584	50,607
Net income	$ 38,194	$ 31,375	$ 121,383	$ 107,080
Income per common share:
Basic (in Dollars per share)	$ 0.41	$ 0.34	$ 1.30	$ 1.15
Diluted (in Dollars per share)	$ 0.40	$ 0.33	$ 1.28	$ 1.12
Weighted average shares outstanding:
Basic (in Shares)	92,689	93,522	93,286	93,429
Diluted (in Shares)	94,355	95,611	95,100	95,791