Revenue and Related Matters - Timing Of Revenue Recognition Liabilities per Segment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Total revenues	$ 973,135	$ 1,070,882	$ 1,992,026	$ 2,041,326
Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	879,194	833,381	1,789,814	1,665,136
Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	93,941	237,501	202,212	376,190
Research
Disaggregation of Revenue [Line Items]
Total revenues	875,329	826,055	1,784,620	1,651,429
Research | Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	810,564	754,267	1,639,776	1,507,065
Research | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	64,765	71,788	144,844	144,364
Conferences
Disaggregation of Revenue [Line Items]
Total revenues	317	141,174	14,187	193,106
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	317	141,174	14,187	193,106
Consulting
Disaggregation of Revenue [Line Items]
Total revenues	97,489	103,653	193,219	196,791
Consulting | Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	68,630	79,114	150,038	158,071
Consulting | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	$ 28,859	$ 24,539	$ 43,181	$ 38,720