Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenues:
Research	$ 1,829,721	$ 1,583,486	$ 1,445,338
Consulting	346,214	327,735	348,396
Events	268,605	251,835	227,707
Total revenues	2,444,540	2,163,056	2,021,441
Costs and expenses:
Cost of services and product development	945,648	839,076	797,933
Selling, general and administrative	1,089,184	962,677	876,067
Depreciation	37,172	33,789	31,186
Amortization of intangibles	24,797	13,342	8,226
Acquisition and integration charges	42,598	26,175	21,867
Total costs and expenses	2,139,399	1,875,059	1,735,279
Operating income	305,141	287,997	286,162
Interest income	2,449	1,766	1,413
Interest expense	(27,565)	(22,548)	(12,300)
Other income (expense), net	8,406	4,996	(592)
Income before income taxes	288,431	272,211	274,683
Provision for income taxes	94,849	96,576	90,917
Net income	$ 193,582	$ 175,635	$ 183,766
Net income per share:
Basic (in Dollars per share)	$ 2.34	$ 2.09	$ 2.06
Diluted (in Dollars per share)	$ 2.31	$ 2.06	$ 2.03
Weighted average shares outstanding:
Basic (in shares)	82,571	83,852	89,337
Diluted (in shares)	83,820	85,056	90,719