Segment Information - Information About Reportable Segments (Details)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) segment	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Segment Reporting Information [Line Items]
Number of operating segments | segment			3
Operating income	$ 116,002,000	$ 86,096,000	$ 164,801,000	$ 77,385,000
Intersegment revenues
Segment Reporting Information [Line Items]
Revenues	0	0	0		$ 0
Reportable segments
Segment Reporting Information [Line Items]
Revenues	1,070,882,000	1,001,336,000	2,041,326,000	1,964,901,000
Gross contribution	687,103,000	645,168,000	1,309,865,000	1,259,983,000
Corporate and other expenses
Segment Reporting Information [Line Items]
Corporate and other expenses	(571,101,000)	(559,072,000)	(1,145,064,000)	(1,182,598,000)
Research | Reportable segments
Segment Reporting Information [Line Items]
Revenues	826,055,000	770,314,000	1,651,429,000	1,534,238,000
Gross contribution	572,297,000	532,910,000	1,147,465,000	1,064,367,000
Conferences | Reportable segments
Segment Reporting Information [Line Items]
Revenues	141,174,000	111,253,000	193,106,000	157,340,000
Gross contribution	80,570,000	63,461,000	99,446,000	79,651,000
Consulting | Reportable segments
Segment Reporting Information [Line Items]
Revenues	103,653,000	96,458,000	196,791,000	179,354,000
Gross contribution	$ 34,236,000	33,693,000	$ 62,954,000	57,817,000
Other | Reportable segments
Segment Reporting Information [Line Items]
Revenues		23,311,000		93,969,000
Gross contribution		$ 15,104,000		$ 58,148,000