Segment Information (Details) - Information About Reportable Segments	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) segment	Jun. 30, 2016 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | segment			4
Segment Reporting Information [Line Items]
Revenues	$ 843,731,000	$ 609,998,000	$ 1,468,900,000	$ 1,167,264,000
Operating (loss) income	(98,388,000)	83,299,000	(44,874,000)	147,728,000
Intersegment revenues
Segment Reporting Information [Line Items]
Revenues	0	0	0	0
Reportable segments
Segment Reporting Information [Line Items]
Revenues	843,731,000	609,998,000	1,468,900,000	1,167,264,000
Gross contribution	499,036,000	383,024,000	887,653,000	733,570,000
Corporate and other expenses
Segment Reporting Information [Line Items]
Corporate and other expenses	(597,424,000)	(299,725,000)	(932,527,000)	(585,842,000)
Research
Segment Reporting Information [Line Items]
Revenues		7,500,000		14,800,000
Gross contribution		4,900,000		9,800,000
Research | Reportable segments
Segment Reporting Information [Line Items]
Revenues	613,732,000	456,690,000	1,125,038,000	904,280,000
Gross contribution	400,571,000	318,621,000	751,684,000	631,630,000
Consulting
Segment Reporting Information [Line Items]
Revenues		(7,500,000)		(14,800,000)
Gross contribution		(4,900,000)		(9,800,000)
Consulting | Reportable segments
Segment Reporting Information [Line Items]
Revenues	91,693,000	86,548,000	170,287,000	164,169,000
Gross contribution	31,433,000	28,341,000	55,370,000	52,895,000
Events | Reportable segments
Segment Reporting Information [Line Items]
Revenues	91,205,000	66,760,000	126,474,000	98,815,000
Gross contribution	49,735,000	36,062,000	63,302,000	49,045,000
Talent Assessment & Other | Reportable segments
Segment Reporting Information [Line Items]
Revenues	47,101,000	0	47,101,000	0
Gross contribution	$ 17,297,000	$ 0	$ 17,297,000	$ 0