Segment Information - Information About Reportable Segments (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) segment	Sep. 30, 2016 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | segment			4
Segment Reporting Information [Line Items]
Revenues	$ 828,085,000	$ 574,059,000	$ 2,296,985,000	$ 1,741,323,000
Operating (loss) income	(24,349,000)	48,726,000	(69,223,000)	196,454,000
Intersegment revenues
Segment Reporting Information [Line Items]
Revenues	0	0	0	0
Reportable segments
Segment Reporting Information [Line Items]
Revenues	828,085,000	574,059,000	2,296,985,000	1,741,323,000
Gross contribution	499,636,000	355,390,000	1,387,289,000	1,088,960,000
Corporate and other expenses
Segment Reporting Information [Line Items]
Corporate and other expenses	(523,985,000)	(306,664,000)	(1,456,512,000)	(892,506,000)
Research
Segment Reporting Information [Line Items]
Revenues		5,400,000		20,200,000
Gross contribution		3,400,000		13,200,000
Research | Reportable segments
Segment Reporting Information [Line Items]
Revenues	653,443,000	466,877,000	1,778,481,000	1,371,157,000
Gross contribution	436,222,000	322,646,000	1,187,906,000	954,276,000
Consulting
Segment Reporting Information [Line Items]
Revenues		(5,400,000)		(20,200,000)
Gross contribution		(3,400,000)		(13,200,000)
Consulting | Reportable segments
Segment Reporting Information [Line Items]
Revenues	72,117,000	73,707,000	242,404,000	237,876,000
Gross contribution	16,188,000	18,215,000	71,558,000	71,110,000
Events | Reportable segments
Segment Reporting Information [Line Items]
Revenues	44,953,000	33,475,000	171,427,000	132,290,000
Gross contribution	16,011,000	14,529,000	79,313,000	63,574,000
Talent Assessment & Other | Reportable segments
Segment Reporting Information [Line Items]
Revenues	57,572,000	0	104,673,000	0
Gross contribution	$ 31,215,000	$ 0	$ 48,512,000	$ 0