Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues:
Research	$ 2,471,280	$ 1,857,001	$ 1,614,904
Consulting	327,661	318,934	296,317
Events	337,903	268,605	251,835
Talent Assessment & Other	174,650	0	0
Total revenues	3,311,494	2,444,540	2,163,056
Costs and expenses:
Cost of services and product development	1,320,198	945,648	839,076
Selling, general and administrative	1,599,004	1,089,184	962,677
Depreciation	63,897	37,172	33,789
Amortization of intangibles	176,274	24,797	13,342
Acquisition and integration charges	158,450	42,598	26,175
Total costs and expenses	3,317,823	2,139,399	1,875,059
Operating (loss) income	(6,329)	305,141	287,997
Interest income	3,011	2,449	1,766
Interest expense	(127,947)	(27,565)	(22,548)
Other income, net	3,448	8,406	4,996
(Loss) income before income taxes	(127,817)	288,431	272,211
(Benefit) provision for income taxes	(131,096)	94,849	96,576
Net income	$ 3,279	$ 193,582	$ 175,635
Net income per share:
Basic (in Dollars per share)	$ 0.04	$ 2.34	$ 2.09
Diluted (in Dollars per share)	$ 0.04	$ 2.31	$ 2.06
Weighted average shares outstanding:
Basic (in shares)	88,466	82,571	83,852
Diluted (in shares)	89,790	83,820	85,056