Segment Information - Reconciliation of Segment Gross Contribution to Net Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenues	$ 1,686,454	$ 1,595,060	$ 3,220,584	$ 3,067,986
Personnel expenses	404,917	380,717	791,116	744,362
Product and content delivery expenses	107,547	104,794	176,157	178,365
Other	16,372	14,597	30,396	28,196
Gross contribution	1,157,618	1,094,952	2,222,915	2,117,063
Cost of services and product development - unallocated	2,895	13,206	9,092	21,832
Selling, general and administrative	776,888	712,071	1,507,196	1,401,904
Depreciation and amortization	50,739	50,534	101,499	99,841
Acquisition and integration charges	0	358	0	818
Interest expenses and other, net	9,303	19,486	20,327	33,814
Income before income taxes	317,793	299,297	584,801	558,854
Insights
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenues	1,319,453	1,265,992	2,641,280	2,534,164
Conferences
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenues	211,407	186,084	284,004	256,153
Consulting
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenues	155,594	142,984	$ 295,300	$ 277,669
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	1,319,453	1,265,992	$ 2,641,280	$ 2,534,164
Personnel expenses	299,525	282,155	588,887	556,744
Product and content delivery expenses	38,298	44,057	79,475	87,644
Other	6,955	6,659	13,720	12,086
Gross contribution	974,675	933,121	1,959,198	1,877,690
Operating Segments | Conferences
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenues	211,407	186,084	284,004	256,153
Personnel expenses	18,881	16,541	36,626	32,224
Product and content delivery expenses	65,197	56,055	89,368	83,848
Other	5,941	5,376	9,240	8,714
Gross contribution	121,388	108,112	148,770	131,367
Operating Segments | Consulting
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenues	155,594	142,984	295,300	277,669
Personnel expenses	86,511	82,021	165,603	155,394
Product and content delivery expenses	4,052	4,682	7,314	6,873
Other	3,476	2,562	7,436	7,396
Gross contribution	$ 61,555	$ 53,719	$ 114,947	$ 108,006