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, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Disaggregation of Revenue [Line Items]
Other revenues	$ 1,327,151	$ 1,278,184	$ 3,928,111	$ 3,807,696
Income (loss) from equity method investments	4,200	(7,100)	(400)	(36,100)
Revenue from contracts with customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	1,230,327	1,197,164	3,657,610	3,499,966
Products [Member] \| Revenue from contracts with customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	585,707	575,277	1,921,773	1,809,276
Services [Member] \| Revenue from contracts with customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	600,416	606,246	1,659,729	1,635,493
Investment management [Member] \| Revenue from contracts with customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	44,204	15,641	76,108	55,197
Program services and other fronting [Member]
Disaggregation of Revenue [Line Items]
Other revenues	39,499	40,033	105,952	103,096
Disposition gain
Disaggregation of Revenue [Line Items]
Other revenues			16,923	107,293
Equity method investments income (loss) [Member]
Disaggregation of Revenue [Line Items]
Income (loss) from equity method investments	4,243	(7,101)	(398)	(36,102)
Other [Member]
Disaggregation of Revenue [Line Items]
Other revenues	53,082	48,088	148,024	133,443
Markel Ventures [Member]
Disaggregation of Revenue [Line Items]
Other revenues	1,245,577	1,215,746	3,735,329	3,527,429
Markel Ventures [Member] \| Revenue from contracts with customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	1,183,219	1,169,994	3,573,960	3,403,222
Markel Ventures [Member] \| Products [Member] \| Revenue from contracts with customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	585,707	575,277	1,921,773	1,809,276
Markel Ventures [Member] \| Services [Member] \| Revenue from contracts with customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	597,512	594,717	1,652,187	1,593,946
Markel Ventures [Member] \| Investment management [Member] \| Revenue from contracts with customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	0	0	0	0
Markel Ventures [Member] \| Program services and other fronting [Member]
Disaggregation of Revenue [Line Items]
Other revenues	0	0	0	0
Markel Ventures [Member] \| Disposition gain
Disaggregation of Revenue [Line Items]
Other revenues			0	0
Markel Ventures [Member] \| Equity method investments income (loss) [Member]
Disaggregation of Revenue [Line Items]
Income (loss) from equity method investments	(21)	(546)	1,087	1,537
Markel Ventures [Member] \| Other [Member]
Disaggregation of Revenue [Line Items]
Other revenues	62,379	46,298	160,282	122,670
Other [Member]
Disaggregation of Revenue [Line Items]
Other revenues	81,574	62,438	192,782	280,267
Other [Member] \| Revenue from contracts with customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	47,108	27,170	83,650	96,744
Other [Member] \| Products [Member] \| Revenue from contracts with customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	0	0	0	0
Other [Member] \| Services [Member] \| Revenue from contracts with customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	2,904	11,529	7,542	41,547
Other [Member] \| Investment management [Member] \| Revenue from contracts with customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	44,204	15,641	76,108	55,197
Other [Member] \| Program services and other fronting [Member]
Disaggregation of Revenue [Line Items]
Other revenues	39,499	40,033	105,952	103,096
Other [Member] \| Disposition gain
Disaggregation of Revenue [Line Items]
Other revenues			16,923	107,293
Other [Member] \| Equity method investments income (loss) [Member]
Disaggregation of Revenue [Line Items]
Income (loss) from equity method investments	4,264	(6,555)	(1,485)	(37,639)
Other [Member] \| Other [Member]
Disaggregation of Revenue [Line Items]
Other revenues	$ (9,297)	$ 1,790	$ (12,258)	$ 10,773