Business and Basis of Presentation - Timing Of Revenue Recognition Liabilities per Segment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Total revenues	$ 970,444	$ 963,565
Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	831,755	834,052
Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	138,689	129,513
Research
Disaggregation of Revenue [Line Items]
Total revenues	825,374	763,924
Research | Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	752,798	701,096
Research | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	72,576	62,828
Conferences
Disaggregation of Revenue [Line Items]
Total revenues	51,932	46,087
Conferences | Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	0	0
Conferences | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	51,932	46,087
Consulting
Disaggregation of Revenue [Line Items]
Total revenues	93,138	82,896
Consulting | Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	78,957	74,010
Consulting | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	$ 14,181	8,886
Other
Disaggregation of Revenue [Line Items]
Total revenues		70,658
Other | Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues		58,946
Other | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues		$ 11,712