Condensed Consolidated Statements of Operations (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Revenues:
Research	$ 311,233	$ 278,302	$ 621,564	$ 552,922
Consulting	85,928	76,676	158,561	151,239
Events	48,886	42,504	72,676	62,492
Total revenues	446,047	397,482	852,801	766,653
Costs and expenses:
Cost of services and product development	177,904	161,247	341,641	307,710
Selling, general and administrative	185,629	165,221	366,107	327,739
Depreciation	7,017	6,182	14,117	12,077
Amortization of intangibles	1,404	928	2,738	1,667
Acquisition and integration charges	106	1,182	206	1,182
Total costs and expenses	372,060	334,760	724,809	650,375
Operating income	73,987	62,722	127,992	116,278
Interest expense, net	(2,144)	(2,153)	(4,580)	(4,348)
Other expense, net	(280)	(76)	(69)	(1,054)
Income before income taxes	71,563	60,493	123,343	110,876
Provision for income taxes	25,049	19,009	40,154	35,171
Net income	$ 46,514	$ 41,484	$ 83,189	$ 75,705
Income per common share:
Basic (in Dollars per share)	$ 0.50	$ 0.44	$ 0.89	$ 0.81
Diluted (in Dollars per share)	$ 0.49	$ 0.43	$ 0.87	$ 0.79
Weighted average shares outstanding:
Basic (in Shares)	93,574	93,350	93,584	93,383
Diluted (in Shares)	95,188	95,423	95,426	95,826