Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenues:
Total revenues	$ 4,245,321	$ 3,975,454	$ 3,311,494
Costs and expenses:
Cost of services and product development	1,550,568	1,468,800	1,320,198
Selling, general and administrative	2,103,424	1,884,141	1,599,004
Depreciation	82,066	68,592	63,897
Amortization of intangibles	129,713	187,009	176,274
Acquisition and integration charges	9,463	107,197	158,450
Total costs and expenses	3,875,234	3,715,739	3,317,823
Operating income (loss)	370,087	259,715	(6,329)
Interest income	3,026	2,566	3,011
Interest expense	(102,831)	(126,774)	(127,947)
(Loss) gain from divested operations	(2,075)	45,447	0
Other income, net	7,532	167	3,448
Income (loss) before income taxes	275,739	181,121	(127,817)
Provision (benefit) for income taxes	42,449	58,665	(131,096)
Net income	$ 233,290	$ 122,456	$ 3,279
Net income per share:
Basic (in dollars per share)	$ 2.60	$ 1.35	$ 0.04
Diluted (in dollars per share)	$ 2.56	$ 1.33	$ 0.04
Weighted average shares outstanding:
Basic (in shares)	89,817	90,827	88,466
Diluted (in shares)	90,971	92,122	89,790
Research
Revenues:
Total revenues	$ 3,374,548	$ 3,105,764	$ 2,471,280
Conferences
Revenues:
Total revenues	476,869	410,461	337,903
Consulting
Revenues:
Total revenues	393,904	353,667	327,661
Other
Revenues:
Total revenues	$ 0	$ 105,562	$ 174,650