Segment Information - Information About Reportable Segments (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) segment	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Segment Reporting Information [Line Items]
Number of operating segments | segment			3
Operating income	$ 69,147,000	$ 52,724,000	$ 233,948,000	$ 130,109,000
Intersegment revenues
Segment Reporting Information [Line Items]
Revenues	0	0	0		$ 0
Reportable segments
Segment Reporting Information [Line Items]
Revenues	1,000,502,000	921,674,000	3,041,828,000	2,886,575,000
Operating income	636,505,000	585,304,000	1,946,370,000	1,845,286,000
Corporate and other expenses
Segment Reporting Information [Line Items]
Operating income	(567,358,000)	(532,580,000)	(1,712,422,000)	(1,715,177,000)
Research | Reportable segments
Segment Reporting Information [Line Items]
Revenues	840,998,000	774,188,000	2,492,427,000	2,308,426,000
Operating income	582,502,000	534,911,000	1,729,967,000	1,599,277,000
Conferences | Reportable segments
Segment Reporting Information [Line Items]
Revenues	66,286,000	57,141,000	259,392,000	214,481,000
Operating income	27,465,000	25,047,000	126,910,000	104,698,000
Consulting | Reportable segments
Segment Reporting Information [Line Items]
Revenues	93,218,000	78,752,000	290,009,000	258,106,000
Operating income	$ 26,538,000	18,419,000	$ 89,493,000	76,236,000
Other | Reportable segments
Segment Reporting Information [Line Items]
Revenues		11,593,000		105,562,000
Operating income		$ 6,927,000		$ 65,075,000