Business and Basis of Presentation - Timing Of Revenue Recognition Liabilities per Segment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Revenue from contracts with customer	$ 963,565	$ 625,169
Transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customer	834,052	531,700
Transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customer	129,513	93,469
Research
Disaggregation of Revenue [Line Items]
Revenue from contracts with customer	763,924	511,306
Research | Transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customer	701,096	466,706
Research | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customer	62,828	44,600
Events
Disaggregation of Revenue [Line Items]
Revenue from contracts with customer	46,087	35,269
Events | Transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customer	0	0
Events | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customer	46,087	35,269
Consulting
Disaggregation of Revenue [Line Items]
Revenue from contracts with customer	82,896	78,594
Consulting | Transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customer	74,010	64,994
Consulting | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customer	8,886	13,600
Talent Assessment & Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customer	70,658	0
Talent Assessment & Other | Transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customer	58,946	0
Talent Assessment & Other | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customer	$ 11,712	$ 0