Segment Information - Information About Reportable Segments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Segment Reporting Information [Line Items]
Operating Income	$ 1,236,894	$ 1,100,106	$ 915,751
Operating Segments
Segment Reporting Information [Line Items]
Revenues	5,906,956	5,475,846	4,733,962
Operating Income	4,035,383	3,815,134	3,329,516
Operating Segments | Research
Segment Reporting Information [Line Items]
Revenues	4,887,046	4,604,791	4,101,392
Operating Income	3,600,143	3,414,574	3,036,925
Operating Segments | Conferences
Segment Reporting Information [Line Items]
Revenues	505,164	389,273	214,449
Operating Income	253,739	210,726	133,748
Operating Segments | Consulting
Segment Reporting Information [Line Items]
Revenues	514,746	481,782	418,121
Operating Income	181,501	189,834	158,843
Corporate and Reconciling Items
Segment Reporting Information [Line Items]
Operating Income	$ (2,798,489)	$ (2,715,028)	$ (2,413,765)