Business and Basis of Presentation - Timing Of Revenue Recognition Liabilities per Segment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Total revenues	$ 1,070,882	$ 1,001,336	$ 2,041,326	$ 1,964,901
Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	833,381	804,795	1,665,136	1,638,847
Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	237,501	196,541	376,190	326,054
Research
Disaggregation of Revenue [Line Items]
Total revenues	826,055	770,314	1,651,429	1,534,238
Research | Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	754,267	708,801	1,507,065	1,409,897
Research | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	71,788	61,513	144,364	124,341
Conferences
Disaggregation of Revenue [Line Items]
Total revenues	141,174	111,253	193,106	157,340
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	141,174	111,253	193,106	157,340
Consulting
Disaggregation of Revenue [Line Items]
Total revenues	103,653	96,458	196,791	179,354
Consulting | Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	79,114	77,073	158,071	151,083
Consulting | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	$ 24,539	19,385	$ 38,720	28,271
Other
Disaggregation of Revenue [Line Items]
Total revenues		23,311		93,969
Other | Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues		18,921		77,867
Other | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues		$ 4,390		$ 16,102