Products, Services and Other Revenues (Schedule Of Revenues From Contracts With Customers By Type) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Other revenues	$ 2,132,606	$ 1,413,275	$ 1,307,779
Revenue from Contract with Customer [Member]
Disaggregation of Revenue [Line Items]
Other revenues	1,944,667	1,305,655	1,223,305
Markel Ventures Operations [Member]
Disaggregation of Revenue [Line Items]
Other revenues	1,912,065	1,333,280	1,214,449
Markel Ventures Operations [Member] | Revenue from Contract with Customer [Member]
Disaggregation of Revenue [Line Items]
Other revenues	1,819,904	1,242,169	1,131,866
Other [Member]
Disaggregation of Revenue [Line Items]
Other revenues	220,541	79,995	93,330
Other [Member] | Revenue from Contract with Customer [Member]
Disaggregation of Revenue [Line Items]
Other revenues	124,763	63,486	91,439
Product [Member] | Revenue from Contract with Customer [Member]
Disaggregation of Revenue [Line Items]
Other revenues	1,452,332	902,739	843,140
Product [Member] | Markel Ventures Operations [Member] | Revenue from Contract with Customer [Member]
Disaggregation of Revenue [Line Items]
Other revenues	1,452,332	902,739	843,140
Product [Member] | Other [Member] | Revenue from Contract with Customer [Member]
Disaggregation of Revenue [Line Items]
Other revenues	0	0	0
Services [Member] | Revenue from Contract with Customer [Member]
Disaggregation of Revenue [Line Items]
Other revenues	400,808	374,176	323,710
Services [Member] | Markel Ventures Operations [Member] | Revenue from Contract with Customer [Member]
Disaggregation of Revenue [Line Items]
Other revenues	367,572	339,430	288,726
Services [Member] | Other [Member] | Revenue from Contract with Customer [Member]
Disaggregation of Revenue [Line Items]
Other revenues	33,236	34,746	34,984
Investment Management [Member] | Revenue from Contract with Customer [Member]
Disaggregation of Revenue [Line Items]
Other revenues	91,527	28,740	56,455
Investment Management [Member] | Markel Ventures Operations [Member] | Revenue from Contract with Customer [Member]
Disaggregation of Revenue [Line Items]
Other revenues	0	0	0
Investment Management [Member] | Other [Member] | Revenue from Contract with Customer [Member]
Disaggregation of Revenue [Line Items]
Other revenues	91,527
Program Services [Member]
Disaggregation of Revenue [Line Items]
Other revenues	94,118	14,487	0
Program Services [Member] | Markel Ventures Operations [Member]
Disaggregation of Revenue [Line Items]
Other revenues	0	0	0
Program Services [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Other revenues	94,118	14,487	0
Other Revenues [Member]
Disaggregation of Revenue [Line Items]
Other revenues	93,821	93,133	84,474
Other Revenues [Member] | Markel Ventures Operations [Member]
Disaggregation of Revenue [Line Items]
Other revenues	92,161	91,111	82,583
Other Revenues [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Other revenues	$ 1,660	$ 2,022	$ 1,891