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, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Disaggregation of Revenue [Line Items]
Leasing revenues	$ 45,726	$ 50,994	$ 123,514	$ 130,051
Program services and other fronting fees	47,613	39,499	129,861	105,952
Equity method and other investments income (loss)	8,677	(5,054)	45,466	(12,704)
Disposition gain			0	16,923
Other revenues (losses)	1,501	13,438	3,987	31,127
Products Revenues and Services and Other Revenues	1,348,149	1,327,151	4,098,327	3,928,111
Markel Ventures [Member]
Disaggregation of Revenue [Line Items]
Leasing revenues	45,726	50,994	123,514	130,051
Program services and other fronting fees	0	0	0	0
Equity method and other investments income (loss)	(6,294)	(21)	292	1,087
Disposition gain			0	0
Other revenues (losses)	1,563	13,438	4,067	31,079
Products Revenues and Services and Other Revenues	1,257,528	1,245,577	3,848,243	3,735,329
Other [Member]
Disaggregation of Revenue [Line Items]
Leasing revenues	0	0	0	0
Program services and other fronting fees	47,613	39,499	129,861	105,952
Equity method and other investments income (loss)	14,971	(5,033)	45,174	(13,791)
Disposition gain			0	16,923
Other revenues (losses)	(62)	0	(80)	48
Products Revenues and Services and Other Revenues	90,621	81,574	250,084	192,782
Revenue from contracts with customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	1,244,632	1,228,274	3,795,499	3,656,762
Revenue from contracts with customers [Member] | Products [Member]
Disaggregation of Revenue [Line Items]
Other revenues	605,471	585,707	2,040,699	1,921,773
Revenue from contracts with customers [Member] | Services [Member]
Disaggregation of Revenue [Line Items]
Other revenues	613,303	598,363	1,686,460	1,658,881
Revenue from contracts with customers [Member] | Management Fees [Member]
Disaggregation of Revenue [Line Items]
Other revenues	25,858	44,204	68,340	76,108
Revenue from contracts with customers [Member] | Markel Ventures [Member]
Disaggregation of Revenue [Line Items]
Other revenues	1,216,533	1,181,166	3,720,370	3,573,112
Revenue from contracts with customers [Member] | Markel Ventures [Member] | Products [Member]
Disaggregation of Revenue [Line Items]
Other revenues	605,471	585,707	2,040,699	1,921,773
Revenue from contracts with customers [Member] | Markel Ventures [Member] | Services [Member]
Disaggregation of Revenue [Line Items]
Other revenues	611,062	595,459	1,679,671	1,651,339
Revenue from contracts with customers [Member] | Markel Ventures [Member] | Management Fees [Member]
Disaggregation of Revenue [Line Items]
Other revenues	0	0	0	0
Revenue from contracts with customers [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Other revenues	28,099	47,108	75,129	83,650
Revenue from contracts with customers [Member] | Other [Member] | Products [Member]
Disaggregation of Revenue [Line Items]
Other revenues	0	0	0	0
Revenue from contracts with customers [Member] | Other [Member] | Services [Member]
Disaggregation of Revenue [Line Items]
Other revenues	2,241	2,904	6,789	7,542
Revenue from contracts with customers [Member] | Other [Member] | Management Fees [Member]
Disaggregation of Revenue [Line Items]
Other revenues	$ 25,858	$ 44,204	$ 68,340	$ 76,108