Revenue and Related Matters - Timing Of Revenue Recognition Liabilities per Segment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Total revenues	$ 1,018,891	$ 970,444
Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	910,620	831,755
Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	108,271	138,689
Research
Disaggregation of Revenue [Line Items]
Total revenues	909,291	825,374
Research | Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	829,212	752,798
Research | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	80,079	72,576
Conferences
Disaggregation of Revenue [Line Items]
Total revenues	13,870	51,932
Conferences | Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	0	0
Conferences | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	13,870	51,932
Consulting
Disaggregation of Revenue [Line Items]
Total revenues	95,730	93,138
Consulting | Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	81,408	78,957
Consulting | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	$ 14,322	$ 14,181