Products, Services and Other Revenues (Schedule Of Revenues From Contracts With Customers By Type) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Disaggregation of Revenue [Line Items]
Leasing revenues	$ 60,156	$ 45,726	$ 168,704	$ 123,514
Fronting fees	70,073	47,613	159,534	129,861
Equity method and other investments income (loss)	34,922	8,677	96,062	45,466
Other revenues (losses)	6,610	1,501	9,372	3,987
Products Revenues and Services and Other Revenues	1,483,101	1,348,149	4,355,651	4,098,327
Markel Insurance
Disaggregation of Revenue [Line Items]
Leasing revenues	0	0	0	0
Fronting fees	29,026	12,627	37,958	19,875
Equity method and other investments income (loss)	12,373	2,798	10,515	9,143
Other revenues (losses)	4,751	67	5,031	118
Products Revenues and Services and Other Revenues	47,349	18,085	58,252	35,449
Industrial
Disaggregation of Revenue [Line Items]
Leasing revenues	43,218	29,843	117,788	76,160
Fronting fees	0	0	0	0
Equity method and other investments income (loss)	0	(86)	350	658
Other revenues (losses)	609	346	1,177	269
Products Revenues and Services and Other Revenues	1,049,961	998,308	2,895,250	2,784,548
Financial
Disaggregation of Revenue [Line Items]
Leasing revenues	0	0	0	0
Fronting fees	41,047	34,986	121,576	109,986
Equity method and other investments income (loss)	1,528	(5,989)	46,701	(3,004)
Other revenues (losses)	0	0	0	0
Products Revenues and Services and Other Revenues	73,356	54,503	255,227	175,798
Consumer and Other
Disaggregation of Revenue [Line Items]
Leasing revenues	16,938	15,883	50,916	47,354
Fronting fees	0	0	0	0
Equity method and other investments income (loss)	0	0	0	0
Other revenues (losses)	1,250	1,088	3,164	3,600
Products Revenues and Services and Other Revenues	291,414	265,299	1,108,426	1,063,863
Corporate Operations
Disaggregation of Revenue [Line Items]
Leasing revenues	0	0	0	0
Fronting fees	0	0	0	0
Equity method and other investments income (loss)	21,021	11,954	38,496	38,669
Other revenues (losses)	0	0	0	0
Products Revenues and Services and Other Revenues	21,021	11,954	38,496	38,669
Revenue from contracts with customers [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	1,311,340	1,244,632	3,921,979	3,795,499
Revenue from contracts with customers [Member] \| Products [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	599,614	605,471	1,993,412	2,040,699
Revenue from contracts with customers [Member] \| Services [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	681,386	613,303	1,842,777	1,686,460
Revenue from contracts with customers [Member] \| Management Fees [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	30,340	25,858	85,790	68,340
Revenue from contracts with customers [Member] \| Markel Insurance
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	1,199	2,593	4,748	6,313
Revenue from contracts with customers [Member] \| Markel Insurance \| Products [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	0	0	0	0
Revenue from contracts with customers [Member] \| Markel Insurance \| Services [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	1,199	2,593	4,748	6,313
Revenue from contracts with customers [Member] \| Markel Insurance \| Management Fees [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	0	0	0	0
Revenue from contracts with customers [Member] \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	1,006,134	968,205	2,775,935	2,707,461
Revenue from contracts with customers [Member] \| Industrial \| Products [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	417,986	432,120	1,197,798	1,261,664
Revenue from contracts with customers [Member] \| Industrial \| Services [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	588,148	536,085	1,578,137	1,445,797
Revenue from contracts with customers [Member] \| Industrial \| Management Fees [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	0	0	0	0
Revenue from contracts with customers [Member] \| Financial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	30,781	25,506	86,950	68,816
Revenue from contracts with customers [Member] \| Financial \| Products [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	0	0	0	0
Revenue from contracts with customers [Member] \| Financial \| Services [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	441	(352)	1,160	476
Revenue from contracts with customers [Member] \| Financial \| Management Fees [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	30,340	25,858	85,790	68,340
Revenue from contracts with customers [Member] \| Consumer and Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	273,226	248,328	1,054,346	1,012,909
Revenue from contracts with customers [Member] \| Consumer and Other \| Products [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	181,628	173,351	795,614	779,035
Revenue from contracts with customers [Member] \| Consumer and Other \| Services [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	91,598	74,977	258,732	233,874
Revenue from contracts with customers [Member] \| Consumer and Other \| Management Fees [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	0	0	0	0
Revenue from contracts with customers [Member] \| Corporate Operations
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	0	0	0	0
Revenue from contracts with customers [Member] \| Corporate Operations \| Products [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	0	0	0	0
Revenue from contracts with customers [Member] \| Corporate Operations \| Services [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	0	0	0	0
Revenue from contracts with customers [Member] \| Corporate Operations \| Management Fees [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	$ 0	$ 0	$ 0	$ 0