Segment Information - Information About Reportable Segments (Details)		3 Months Ended		6 Months Ended
	Apr. 05, 2017 business	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) segment	Jun. 30, 2017 USD ($) segment
Segment Reporting [Abstract]
Number of business acquired | business	2
Segment Reporting Information [Line Items]
Number of operating segments | segment				4
Number of reportable segments | segment				4	4,000
Operating income (loss)		$ 86,096,000	$ (98,388,000)	$ 77,385,000	$ (44,874,000)
Intersegment revenues
Segment Reporting Information [Line Items]
Revenues		0	0	0	0
Reportable segments
Segment Reporting Information [Line Items]
Revenues		1,001,336,000	843,731,000	1,964,901,000	1,468,900,000
Gross contribution		645,168,000	499,036,000	1,259,983,000	887,653,000
Corporate and other expenses
Segment Reporting Information [Line Items]
Corporate and other expenses		(559,072,000)	(597,424,000)	(1,182,598,000)	(932,527,000)
Research | Reportable segments
Segment Reporting Information [Line Items]
Revenues		770,314,000	613,732,000	1,534,238,000	1,125,038,000
Gross contribution		532,910,000	400,571,000	1,064,367,000	751,684,000
Events | Reportable segments
Segment Reporting Information [Line Items]
Revenues		111,253,000	91,205,000	157,340,000	126,474,000
Gross contribution		63,461,000	49,735,000	79,651,000	63,302,000
Consulting | Reportable segments
Segment Reporting Information [Line Items]
Revenues		96,458,000	91,693,000	179,354,000	170,287,000
Gross contribution		33,693,000	31,433,000	57,817,000	55,370,000
Other | Reportable segments
Segment Reporting Information [Line Items]
Revenues		23,311,000	47,101,000	93,969,000	47,101,000
Gross contribution		$ 15,104,000	$ 17,297,000	$ 58,148,000	$ 17,297,000