Business and Basis of Presentation - Timing Of Revenue Recognition Liabilities per Segment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation of Revenue [Line Items]
Total revenues	$ 1,001,336	$ 843,731	$ 1,964,901	$ 1,468,900
Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	804,795	673,883	1,638,847	1,205,583
Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	196,541	169,848	326,054	263,317
Research
Disaggregation of Revenue [Line Items]
Total revenues	770,314	613,732	1,534,238	1,125,038
Research | Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	708,801	570,024	1,409,897	1,036,730
Research | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	61,513	43,708	124,341	88,308
Events
Disaggregation of Revenue [Line Items]
Total revenues	111,253	91,205	157,340	126,474
Events | Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0	0
Events | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	111,253	91,205	157,340	126,474
Consulting
Disaggregation of Revenue [Line Items]
Total revenues	96,458	91,693	179,354	170,287
Consulting | Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	77,073	68,383	151,083	133,377
Consulting | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	19,385	23,310	28,271	36,910
Other
Disaggregation of Revenue [Line Items]
Total revenues	23,311	47,101	93,969	47,101
Other | Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	18,921	35,476	77,867	35,476
Other | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	$ 4,390	$ 11,625	$ 16,102	$ 11,625