Products, Services and Other Revenues (Schedule Of Revenues From Contracts With Customers By Type) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Other Revenues	$ 2,422,788	$ 2,132,606	$ 1,413,275
Program Services and Other Fronting [Member]
Disaggregation of Revenue [Line Items]
Other Revenues	116,376	94,118	14,487
Other [Member]
Disaggregation of Revenue [Line Items]
Other Revenues	107,156	93,821	93,133
Other [Member]
Disaggregation of Revenue [Line Items]
Other Revenues	368,504	220,541	79,995
Other [Member] | Program Services and Other Fronting [Member]
Disaggregation of Revenue [Line Items]
Other Revenues	116,376	94,118	14,487
Other [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Other Revenues	3,817	1,660	2,022
Markel Ventures [Member]
Disaggregation of Revenue [Line Items]
Other Revenues	2,054,284	1,912,065	1,333,280
Markel Ventures [Member] | Program Services and Other Fronting [Member]
Disaggregation of Revenue [Line Items]
Other Revenues	0	0	0
Markel Ventures [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Other Revenues	103,339	92,161	91,111
Revenue from Contract with Customer Benchmark [Member]
Disaggregation of Revenue [Line Items]
Other Revenues	2,199,256	1,944,667	1,305,655
Revenue from Contract with Customer Benchmark [Member] | Products [Member]
Disaggregation of Revenue [Line Items]
Other Revenues	1,558,265	1,452,332	902,739
Revenue from Contract with Customer Benchmark [Member] | Services [Member]
Disaggregation of Revenue [Line Items]
Other Revenues	490,127	400,808	374,176
Revenue from Contract with Customer Benchmark [Member] | Investment Management [Member]
Disaggregation of Revenue [Line Items]
Other Revenues	150,864	91,527	28,740
Revenue from Contract with Customer Benchmark [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Other Revenues	248,311	124,763	63,486
Revenue from Contract with Customer Benchmark [Member] | Other [Member] | Products [Member]
Disaggregation of Revenue [Line Items]
Other Revenues	0	0	0
Revenue from Contract with Customer Benchmark [Member] | Other [Member] | Services [Member]
Disaggregation of Revenue [Line Items]
Other Revenues	97,447	33,236	34,746
Revenue from Contract with Customer Benchmark [Member] | Other [Member] | Investment Management [Member]
Disaggregation of Revenue [Line Items]
Other Revenues	150,864	91,527	28,740
Revenue from Contract with Customer Benchmark [Member] | Markel Ventures [Member]
Disaggregation of Revenue [Line Items]
Other Revenues	1,950,945	1,819,904	1,242,169
Revenue from Contract with Customer Benchmark [Member] | Markel Ventures [Member] | Products [Member]
Disaggregation of Revenue [Line Items]
Other Revenues	1,558,265	1,452,332	902,739
Revenue from Contract with Customer Benchmark [Member] | Markel Ventures [Member] | Services [Member]
Disaggregation of Revenue [Line Items]
Other Revenues	392,680	367,572	339,430
Revenue from Contract with Customer Benchmark [Member] | Markel Ventures [Member] | Investment Management [Member]
Disaggregation of Revenue [Line Items]
Other Revenues	$ 0	$ 0	$ 0