Segment Information - Information About Reportable Segments (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2024 USD ($)	Jun. 30, 2023 USD ($)	Jun. 30, 2024 USD ($) segment	Jun. 30, 2023 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment			3
Operating income	$ 318,783,000	$ 283,107,000	$ 592,668,000	$ 691,064,000
Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenues			0
Operating Segments
Segment Reporting Information [Line Items]
Revenues	1,595,060,000	1,503,185,000	3,067,986,000	2,912,054,000
Operating income	1,094,952,000	1,031,053,000	2,117,063,000	2,008,163,000
Operating Segments | Research
Segment Reporting Information [Line Items]
Revenues	1,265,992,000	1,207,885,000	2,534,164,000	2,425,076,000
Operating income	933,121,000	885,282,000	1,877,690,000	1,784,796,000
Operating Segments | Conferences
Segment Reporting Information [Line Items]
Revenues	186,084,000	168,897,000	256,153,000	233,539,000
Operating income	108,112,000	98,450,000	131,367,000	125,238,000
Operating Segments | Consulting
Segment Reporting Information [Line Items]
Revenues	142,984,000	126,403,000	277,669,000	253,439,000
Operating income	53,719,000	47,321,000	108,006,000	98,129,000
Corporate and Reconciling Items
Segment Reporting Information [Line Items]
Operating income	$ (776,169,000)	$ (747,946,000)	$ (1,524,395,000)	$ (1,317,099,000)