Products, Services and Other Revenues (Schedule Of Revenues From Contracts With Customers By Type) (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2026	Mar. 31, 2025
Disaggregation of Revenue [Line Items]
Leasing revenues	$ 52,606	$ 53,648
Fronting fees	48,621	39,654
Equity method and other investments income (loss)	(7,716)	36,400
Other revenues (losses)	301	1,270
Products Revenues and Services and Other Revenues	1,245,829	1,221,707
Markel Insurance
Disaggregation of Revenue [Line Items]
Leasing revenues	0	0
Fronting fees	5,451	3,144
Equity method and other investments income (loss)	(2,279)	(2,516)
Other revenues (losses)	(1,987)	47
Products Revenues and Services and Other Revenues	2,727	2,620
Industrial
Disaggregation of Revenue [Line Items]
Leasing revenues	35,020	36,990
Fronting fees	0	0
Equity method and other investments income (loss)	0	46
Other revenues (losses)	1,285	287
Products Revenues and Services and Other Revenues	883,058	829,574
Financial
Disaggregation of Revenue [Line Items]
Leasing revenues	0	0
Fronting fees	43,170	36,510
Equity method and other investments income (loss)	(12,131)	33,837
Other revenues (losses)	0	9
Products Revenues and Services and Other Revenues	72,853	96,694
Consumer and Other
Disaggregation of Revenue [Line Items]
Leasing revenues	17,586	16,658
Fronting fees	0	0
Equity method and other investments income (loss)	0	0
Other revenues (losses)	1,003	927
Products Revenues and Services and Other Revenues	280,497	287,786
Corporate Operations
Disaggregation of Revenue [Line Items]
Leasing revenues	0	0
Fronting fees	0	0
Equity method and other investments income (loss)	6,694	5,033
Other revenues (losses)	0	0
Products Revenues and Services and Other Revenues	6,694	5,033
Revenue from contracts with customers [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	1,152,017	1,090,735
Revenue from contracts with customers [Member] \| Products [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	564,038	554,117
Revenue from contracts with customers [Member] \| Services [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	546,425	510,750
Revenue from contracts with customers [Member] \| Management Fees [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	41,554	25,868
Revenue from contracts with customers [Member] \| Markel Insurance
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	1,542	1,945
Revenue from contracts with customers [Member] \| Markel Insurance \| Products [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	0	0
Revenue from contracts with customers [Member] \| Markel Insurance \| Services [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	1,542	1,945
Revenue from contracts with customers [Member] \| Markel Insurance \| Management Fees [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	0	0
Revenue from contracts with customers [Member] \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	846,753	792,251
Revenue from contracts with customers [Member] \| Industrial \| Products [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	389,488	362,357
Revenue from contracts with customers [Member] \| Industrial \| Services [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	457,265	429,894
Revenue from contracts with customers [Member] \| Industrial \| Management Fees [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	0	0
Revenue from contracts with customers [Member] \| Financial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	41,814	26,338
Revenue from contracts with customers [Member] \| Financial \| Products [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	0	0
Revenue from contracts with customers [Member] \| Financial \| Services [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	260	470
Revenue from contracts with customers [Member] \| Financial \| Management Fees [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	41,554	25,868
Revenue from contracts with customers [Member] \| Consumer and Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	261,908	270,201
Revenue from contracts with customers [Member] \| Consumer and Other \| Products [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	174,550	191,760
Revenue from contracts with customers [Member] \| Consumer and Other \| Services [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	87,358	78,441
Revenue from contracts with customers [Member] \| Consumer and Other \| Management Fees [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	0	0
Revenue from contracts with customers [Member] \| Corporate Operations
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	0	0
Revenue from contracts with customers [Member] \| Corporate Operations \| Products [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	0	0
Revenue from contracts with customers [Member] \| Corporate Operations \| Services [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	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