Business and Significant Accounting Policies - Timing Of Revenue Recognition Liabilities per Segment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Total revenues	$ 3,975,454	$ 3,311,494	$ 2,444,540
Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	3,232,240	2,686,428	1,978,595
Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	743,214	625,066	465,945
Research
Disaggregation of Revenue [Line Items]
Total revenues	3,105,764	2,471,280	1,857,001
Research | Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	2,851,176	2,275,377	1,710,786
Research | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	254,588	195,903	146,215
Conferences
Disaggregation of Revenue [Line Items]
Total revenues	410,461	337,903	268,605
Conferences | Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0
Conferences | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	410,461	337,903	268,605
Consulting
Disaggregation of Revenue [Line Items]
Total revenues	353,667	327,661	318,934
Consulting | Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	294,397	269,720	267,809
Consulting | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	59,270	57,941	51,125
Other
Disaggregation of Revenue [Line Items]
Total revenues	105,562	174,650	0
Other | Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	86,667	141,331	0
Other | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	$ 18,895	$ 33,319	$ 0