Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenues:
Research	$ 1,583,486	$ 1,445,338	$ 1,271,011
Consulting	327,735	348,396	314,257
Events	251,835	227,707	198,945
Total revenues	2,163,056	2,021,441	1,784,213
Costs and expenses:
Cost of services and product development	839,076	797,933	713,484
Selling, general and administrative	962,677	876,067	760,458
Depreciation	33,789	31,186	28,996
Amortization of intangibles	13,342	8,226	5,446
Acquisition and integration charges	26,175	21,867	337
Total costs and expenses	1,875,059	1,735,279	1,508,721
Operating income	287,997	286,162	275,492
Interest income	1,766	1,413	1,551
Interest expense	(22,548)	(12,300)	(10,388)
Other income (expense), net	4,996	(592)	(216)
Income before income taxes	272,211	274,683	266,439
Provision for income taxes	96,576	90,917	83,638
Net income	$ 175,635	$ 183,766	$ 182,801
Net income per share:
Basic (in Dollars per share)	$ 2.09	$ 2.06	$ 1.97
Diluted (in Dollars per share)	$ 2.06	$ 2.03	$ 1.93
Weighted average shares outstanding:
Basic (in shares)	83,852	89,337	93,015
Diluted (in shares)	85,056	90,719	94,830