Condensed Consolidated Statements of Operations (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Revenues:
Research	$ 385,718	$ 358,495	$ 766,808	$ 706,609
Consulting	88,336	93,488	165,128	177,759
Events	73,882	67,837	87,186	82,154
Total revenues	547,936	519,820	1,019,122	966,522
Costs and expenses:
Cost of services and product development	210,495	203,178	388,257	373,999
Selling, general and administrative	237,991	218,537	467,513	423,154
Depreciation	8,440	7,721	16,429	15,180
Amortization of intangibles	2,107	1,979	4,246	3,258
Acquisition and integration charges	3,683	6,644	8,775	10,000
Total costs and expenses	462,716	438,059	885,220	825,591
Operating income	85,220	81,761	133,902	140,931
Interest expense, net	(5,240)	(2,680)	(8,720)	(4,930)
Other (expense) income, net	(468)	175	(1,430)	(54)
Income before income taxes	79,512	79,256	123,752	135,947
Provision for income taxes	28,357	26,216	44,246	45,171
Net income	$ 51,155	$ 53,040	$ 79,506	$ 90,776
Earnings per common share:
Basic (in Dollars per share)	$ 0.61	$ 0.59	$ 0.94	$ 1.00
Diluted (in Dollars per share)	$ 0.61	$ 0.58	$ 0.92	$ 0.99
Weighted average shares outstanding:
Basic (in shares)	83,203	89,521	84,871	90,595
Diluted (in shares)	84,271	90,744	86,064	92,012