Segment Information - Reconciliation of Segment Gross Contribution to Net Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenues	$ 6,497,226	$ 6,267,411	$ 5,906,956
Personnel expenses	1,570,631	1,517,910	1,436,603
Product and content delivery expenses	395,916	410,223	380,887
Other expenses	62,463	61,734	54,083
Gross contribution	4,468,216	4,277,544	4,035,383
Cost of services and product development - unallocated	24,565	33,155	31,667
Selling, general and administrative	3,067,631	2,884,814	2,701,542
Depreciation and amortization	200,309	202,315	191,103
Acquisition and integration charges	0	973	9,587
Goodwill impairment	150,000	0	0
Gain from sale of divested operation	0	0	(135,410)
Interest expenses and other, net	57,593	68,913	92,842
Gain on event cancellation insurance claims	0	(300,000)	(3,077)
Income before income taxes	$ 968,118	$ 1,387,374	$ 1,147,129
Maximum
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Percent of target bonus charges allocated to segments	100.00%	100.00%	100.00%
Insights
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenues	$ 5,072,570	$ 4,829,051	$ 4,516,035
Goodwill impairment	0
Conferences
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenues	644,743	583,224	505,164
Goodwill impairment	0
Consulting
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenues	552,499	558,537	514,746
Goodwill impairment	0
Other
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenues	227,414	296,599	371,011
Goodwill impairment	150,000
Operating Segments | Insights
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenues	5,072,570	4,829,051	4,516,035
Personnel expenses	1,128,600	1,080,226	1,023,053
Product and content delivery expenses	28,215	27,452	29,821
Other expenses	25,570	24,540	19,430
Gross contribution	3,890,185	3,696,833	3,443,731
Operating Segments | Conferences
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenues	644,743	583,224	505,164
Personnel expenses	73,647	67,326	56,708
Product and content delivery expenses	229,081	215,664	178,675
Other expenses	19,171	18,825	16,042
Gross contribution	322,844	281,409	253,739
Operating Segments | Consulting
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenues	552,499	558,537	514,746
Personnel expenses	334,762	324,890	308,580
Product and content delivery expenses	15,518	14,775	8,978
Other expenses	15,789	15,580	15,687
Gross contribution	186,430	203,292	181,501
Operating Segments | Other
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenues	227,414	296,599	371,011
Personnel expenses	33,622	45,468	48,262
Product and content delivery expenses	123,102	152,332	163,413
Other expenses	1,933	2,789	2,924
Gross contribution	$ 68,757	$ 96,010	$ 156,412