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, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Disaggregation of Revenue [Line Items]
Other revenues	$ 1,457,173	$ 1,416,951	$ 2,600,960	$ 2,529,512
Income (loss) from equity method investments	(7,100)	(4,200)	(8,700)	(27,400)
Revenue from contracts with customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	1,362,413	1,354,917	2,427,283	2,302,802
Products [Member] \| Revenue from contracts with customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	770,171	764,543	1,336,066	1,233,999
Services [Member] \| Revenue from contracts with customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	570,797	570,046	1,059,313	1,029,247
Investment management [Member] \| Revenue from contracts with customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	21,445	20,328	31,904	39,556
Program services and other fronting [Member]
Disaggregation of Revenue [Line Items]
Other revenues	37,563	29,731	66,453	63,063
Disposition gain
Disaggregation of Revenue [Line Items]
Other revenues	16,923	0	16,923	107,293
Equity method investments income (loss) [Member]
Disaggregation of Revenue [Line Items]
Income (loss) from equity method investments	(7,149)	(4,222)	(8,667)	(27,410)
Other [Member]
Disaggregation of Revenue [Line Items]
Other revenues	47,423	36,525	98,968	83,764
Markel Ventures [Member]
Disaggregation of Revenue [Line Items]
Other revenues	1,385,813	1,361,298	2,489,752	2,311,683
Markel Ventures [Member] \| Revenue from contracts with customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	1,339,184	1,326,139	2,390,741	2,233,228
Markel Ventures [Member] \| Products [Member] \| Revenue from contracts with customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	770,171	764,543	1,336,066	1,233,999
Markel Ventures [Member] \| Services [Member] \| Revenue from contracts with customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	569,013	561,596	1,054,675	999,229
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	0	0
Markel Ventures [Member] \| Equity method investments income (loss) [Member]
Disaggregation of Revenue [Line Items]
Income (loss) from equity method investments	(387)	(975)	1,108	2,083
Markel Ventures [Member] \| Other [Member]
Disaggregation of Revenue [Line Items]
Other revenues	47,016	36,134	97,903	76,372
Other [Member]
Disaggregation of Revenue [Line Items]
Other revenues	71,360	55,653	111,208	217,829
Other [Member] \| Revenue from contracts with customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	23,229	28,778	36,542	69,574
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	1,784	8,450	4,638	30,018
Other [Member] \| Investment management [Member] \| Revenue from contracts with customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	21,445	20,328	31,904	39,556
Other [Member] \| Program services and other fronting [Member]
Disaggregation of Revenue [Line Items]
Other revenues	37,563	29,731	66,453	63,063
Other [Member] \| Disposition gain
Disaggregation of Revenue [Line Items]
Other revenues	16,923	0	16,923	107,293
Other [Member] \| Equity method investments income (loss) [Member]
Disaggregation of Revenue [Line Items]
Income (loss) from equity method investments	(6,762)	(3,247)	(9,775)	(29,493)
Other [Member] \| Other [Member]
Disaggregation of Revenue [Line Items]
Other revenues	$ 407	$ 391	$ 1,065	$ 7,392