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, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Disaggregation of Revenue [Line Items]
Leasing revenues	$ 68,920	$ 60,156	$ 177,540	$ 168,704
Fronting fees	53,505	70,073	151,650	159,534
Equity method and other investments income (loss)	31,326	34,922	38,354	96,062
Other revenues (losses)	17,588	6,610	16,157	9,372
Products Revenues and Services and Other Revenues	1,624,580	1,483,101	4,560,945	4,355,651
Markel Insurance
Disaggregation of Revenue [Line Items]
Leasing revenues	0	0	0	0
Fronting fees	12,539	29,026	27,058	37,958
Equity method and other investments income (loss)	6,997	12,373	7,522	10,515
Other revenues (losses)	(2,577)	4,751	(7,687)	5,031
Products Revenues and Services and Other Revenues	20,095	47,349	33,152	58,252
Industrial
Disaggregation of Revenue [Line Items]
Leasing revenues	51,085	43,218	124,254	117,788
Fronting fees	0	0	0	0
Equity method and other investments income (loss)	0	0	0	350
Other revenues (losses)	288	609	1,915	1,177
Products Revenues and Services and Other Revenues	1,181,602	1,049,961	3,102,728	2,895,250
Financial
Disaggregation of Revenue [Line Items]
Leasing revenues	0	0	0	0
Fronting fees	40,966	41,047	124,592	121,576
Equity method and other investments income (loss)	2,726	1,528	(5,169)	46,701
Other revenues (losses)	0	0	0	0
Products Revenues and Services and Other Revenues	85,965	73,356	241,278	255,227
Consumer and Other
Disaggregation of Revenue [Line Items]
Leasing revenues	17,835	16,938	53,286	50,916
Fronting fees	0	0	0	0
Equity method and other investments income (loss)	0	0	0	0
Other revenues (losses)	871	1,250	2,923	3,164
Products Revenues and Services and Other Revenues	296,309	291,414	1,128,780	1,108,426
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)	21,603	21,021	36,001	38,496
Other revenues (losses)	19,006	0	19,006	0
Products Revenues and Services and Other Revenues	40,609	21,021	55,007	38,496
Revenue from contracts with customers [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	1,453,241	1,311,340	4,177,244	3,921,979
Revenue from contracts with customers [Member] \| Products [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	658,929	599,614	2,074,927	1,993,412
Revenue from contracts with customers [Member] \| Services [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	752,114	681,386	1,980,907	1,842,777
Revenue from contracts with customers [Member] \| Management Fees [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	42,198	30,340	121,410	85,790
Revenue from contracts with customers [Member] \| Markel Insurance
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	3,136	1,199	6,259	4,748
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	3,136	1,199	6,259	4,748
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	1,130,229	1,006,134	2,976,559	2,775,935
Revenue from contracts with customers [Member] \| Industrial \| Products [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	478,881	417,986	1,282,831	1,197,798
Revenue from contracts with customers [Member] \| Industrial \| Services [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	651,348	588,148	1,693,728	1,578,137
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	42,273	30,781	121,855	86,950
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	75	441	445	1,160
Revenue from contracts with customers [Member] \| Financial \| Management Fees [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	42,198	30,340	121,410	85,790
Revenue from contracts with customers [Member] \| Consumer and Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	277,603	273,226	1,072,571	1,054,346
Revenue from contracts with customers [Member] \| Consumer and Other \| Products [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	180,048	181,628	792,096	795,614
Revenue from contracts with customers [Member] \| Consumer and Other \| Services [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	97,555	91,598	280,475	258,732
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