Products, Services and Other Revenues (Schedule Of Revenues From Contracts With Customers By Type) (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Disaggregation of Revenue [Line Items]
Leasing revenues	$ 56,014	$ 54,900	$ 108,620	$ 108,548
Fronting fees	49,524	49,807	98,145	89,461
Equity method and other investments income (loss)	14,744	24,740	7,028	61,140
Other revenues (losses)	(1,732)	1,492	(1,431)	2,762
Products Revenues and Services and Other Revenues	1,690,536	1,650,843	2,936,365	2,872,550
Markel Insurance
Disaggregation of Revenue [Line Items]
Leasing revenues	0	0	0	0
Fronting fees	9,068	5,788	14,519	8,932
Equity method and other investments income (loss)	2,804	658	525	(1,858)
Other revenues (losses)	(3,123)	233	(5,110)	280
Products Revenues and Services and Other Revenues	10,330	8,283	13,057	10,903
Industrial
Disaggregation of Revenue [Line Items]
Leasing revenues	38,149	37,580	73,169	74,570
Fronting fees	0	0	0	0
Equity method and other investments income (loss)	0	304	0	350
Other revenues (losses)	342	281	1,627	568
Products Revenues and Services and Other Revenues	1,038,068	1,015,715	1,921,126	1,845,289
Financial
Disaggregation of Revenue [Line Items]
Leasing revenues	0	0	0	0
Fronting fees	40,456	44,019	83,626	80,529
Equity method and other investments income (loss)	4,236	11,336	(7,895)	45,173
Other revenues (losses)	0	(9)	0	0
Products Revenues and Services and Other Revenues	82,460	85,177	155,313	181,871
Consumer and Other
Disaggregation of Revenue [Line Items]
Leasing revenues	17,865	17,320	35,451	33,978
Fronting fees	0	0	0	0
Equity method and other investments income (loss)	0	0	0	0
Other revenues (losses)	1,049	987	2,052	1,914
Products Revenues and Services and Other Revenues	551,974	529,226	832,471	817,012
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)	7,704	12,442	14,398	17,475
Other revenues (losses)	0	0	0	0
Products Revenues and Services and Other Revenues	7,704	12,442	14,398	17,475
Revenue from contracts with customers [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	1,571,986	1,519,904	2,724,003	2,610,639
Revenue from contracts with customers [Member] \| Products [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	851,960	839,681	1,415,998	1,393,798
Revenue from contracts with customers [Member] \| Services [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	682,368	650,641	1,228,793	1,161,391
Revenue from contracts with customers [Member] \| Management Fees [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	37,658	29,582	79,212	55,450
Revenue from contracts with customers [Member] \| Markel Insurance
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	1,581	1,604	3,123	3,549
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	1,581	1,604	3,123	3,549
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	999,577	977,550	1,846,330	1,769,801
Revenue from contracts with customers [Member] \| Industrial \| Products [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	414,462	417,455	803,950	779,812
Revenue from contracts with customers [Member] \| Industrial \| Services [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	585,115	560,095	1,042,380	989,989
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	37,768	29,831	79,582	56,169
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	110	249	370	719
Revenue from contracts with customers [Member] \| Financial \| Management Fees [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	37,658	29,582	79,212	55,450
Revenue from contracts with customers [Member] \| Consumer and Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	533,060	510,919	794,968	781,120
Revenue from contracts with customers [Member] \| Consumer and Other \| Products [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	437,498	422,226	612,048	613,986
Revenue from contracts with customers [Member] \| Consumer and Other \| Services [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	95,562	88,693	182,920	167,134
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