Segment Information - Reconciliation of Segment Gross Contribution to Net Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenues	$ 1,524,072	$ 1,484,306	$ 4,744,656	$ 4,552,292
Personnel expenses	387,788	378,427	1,178,905	1,122,789
Product and content delivery expenses	65,961	76,412	242,118	254,778
Other expenses	13,397	14,035	43,792	42,230
Gross contribution	1,056,926	1,015,432	3,279,841	3,132,495
Cost of services and product development - unallocated	7,072	6,468	16,164	28,300
Selling, general and administrative	762,557	711,729	2,269,753	2,113,633
Depreciation and amortization	50,953	51,252	152,452	151,093
Goodwill impairment	150,000	0	150,000	0
Acquisition and integration charges	0	159	0	977
Interest expenses and other, net	16,862	18,952	37,189	52,766
Gain on event cancellation insurance claims	0	(300,000)	0	(300,000)
Income before income taxes	69,482	526,872	654,283	1,085,726
Insights
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenues	1,270,724	1,209,524	3,789,798	3,582,238
Goodwill impairment			0
Conferences
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenues	74,554	75,776	358,558	331,929
Goodwill impairment			0
Consulting
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenues	123,573	127,622	418,873	405,291
Goodwill impairment			0
Other
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenues	55,221	71,384	177,427	$ 232,834
Goodwill impairment			$ 150,000
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%
Operating Segments | Insights
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenues	1,270,724	1,209,524	$ 3,789,798	$ 3,582,238
Personnel expenses	282,516	271,718	850,941	804,736
Product and content delivery expenses	8,102	7,427	19,429	17,466
Other expenses	5,783	6,607	18,426	17,325
Gross contribution	974,323	923,772	2,901,002	2,742,711
Operating Segments | Conferences
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenues	74,554	75,776	358,558	331,929
Personnel expenses	18,389	16,621	55,015	48,844
Product and content delivery expenses	25,533	25,765	114,901	109,614
Other expenses	3,320	2,901	12,560	11,615
Gross contribution	27,312	30,489	176,082	161,856
Operating Segments | Consulting
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenues	123,573	127,622	418,873	405,291
Personnel expenses	81,040	78,406	246,644	233,800
Product and content delivery expenses	3,588	3,971	10,902	10,844
Other expenses	3,723	3,728	11,158	11,124
Gross contribution	35,222	41,517	150,169	149,523
Operating Segments | Other
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenues	55,221	71,384	177,427	232,834
Personnel expenses	5,843	11,682	26,305	35,409
Product and content delivery expenses	28,738	39,249	96,886	116,854
Other expenses	571	799	1,648	2,166
Gross contribution	$ 20,069	$ 19,654	$ 52,588	$ 78,405