Business and Basis of Presentation - Timing Of Revenue Recognition Liabilities per Segment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Total revenues	$ 921,674	$ 828,085	$ 2,886,575	$ 2,296,985
Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	788,051	711,840	2,426,898	1,917,423
Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	133,623	116,245	459,677	379,562
Research
Disaggregation of Revenue [Line Items]
Total revenues	774,188	653,443	2,308,426	1,778,481
Research | Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	709,496	600,265	2,119,393	1,636,995
Research | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	64,692	53,178	189,033	141,486
Events
Disaggregation of Revenue [Line Items]
Total revenues	57,141	44,953	214,481	171,427
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	57,141	44,953	214,481	171,427
Consulting
Disaggregation of Revenue [Line Items]
Total revenues	78,752	72,117	258,106	242,404
Consulting | Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	69,755	64,565	220,838	197,942
Consulting | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	8,997	7,552	37,268	44,462
Other
Disaggregation of Revenue [Line Items]
Total revenues	11,593	57,572	105,562	104,673
Other | Transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	8,800	47,010	86,667	82,486
Other | Transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	$ 2,793	$ 10,562	$ 18,895	$ 22,187