Business and Basis of Presentation - Timing Of Revenue Recognition Liabilities per Segment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Total revenues	$ 1,000,502	$ 921,674	$ 3,041,828	$ 2,886,575
Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	847,288	788,051	2,512,424	2,426,898
Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	153,214	133,623	529,404	459,677
Research
Disaggregation of Revenue [Line Items]
Total revenues	840,998	774,188	2,492,427	2,308,426
Research | Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	769,718	709,496	2,276,783	2,119,393
Research | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	71,280	64,692	215,644	189,033
Conferences
Disaggregation of Revenue [Line Items]
Total revenues	66,286	57,141	259,392	214,481
Conferences | Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0	0
Conferences | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	66,286	57,141	259,392	214,481
Consulting
Disaggregation of Revenue [Line Items]
Total revenues	93,218	78,752	290,009	258,106
Consulting | Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	77,570	69,755	235,641	220,838
Consulting | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	$ 15,648	8,997	$ 54,368	37,268
Other
Disaggregation of Revenue [Line Items]
Total revenues		11,593		105,562
Other | Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues		8,800		86,667
Other | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues		$ 2,793		$ 18,895