Consolidated Statements of Operations (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Revenues:
Research	$ 1,137,147		$ 1,012,062		$ 865,000
Consulting	304,893		308,047		302,117
Events	173,768		148,479		121,337
Total revenues	1,615,808		1,468,588		1,288,454
Costs and expenses:
Cost of services and product development	659,067		608,755		552,238
Selling, general and administrative	678,843		613,707		543,174
Depreciation	25,369		25,539		25,349
Amortization of intangibles	4,402		6,525		10,525
Acquisition and integration charges	2,420				7,903
Total costs and expenses	1,370,101		1,254,526		1,139,189
Operating income	245,707		214,062		149,265
Interest income	1,046		1,249		1,156
Interest expense	(9,905)		(11,216)		(16,772)
Other (expense) income, net	(1,252)		(1,911)		436
Income before income taxes	235,596		202,184		134,085
Provision for income taxes	69,693		65,282		37,800
Net income	$ 165,903		$ 136,902		$ 96,285
Net income per share:
Basic (in Dollars per share)	$ 1.78		$ 1.43		$ 1.01
Diluted (in Dollars per share)	$ 1.73		$ 1.39		$ 0.96
Weighted average shares outstanding:
Basic (in Shares)	93,444	[1]	96,019	[1]	95,747	[1]
Diluted (in Shares)	95,842	[1]	98,846	[1]	99,834	[1]

[1] During 2012, 2011 and 2010, the Company repurchased 2.7 million, 5.9 million, and 3.9 million shares of its Common Stock, respectively.