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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Other revenues	$ 587,500	$ 518,244	$ 1,831,809	$ 1,585,869
Revenue from contracts with customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	535,749	472,151	1,667,621	1,450,711
Products [Member] | Revenue from contracts with customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	378,421	350,981	1,198,124	1,095,461
Services [Member] | Revenue from contracts with customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	114,578	102,912	355,759	302,285
Investment management [Member] | Revenue from contracts with customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	42,750	18,258	113,738	52,965
Program services and other fronting arrangements [Member]
Disaggregation of Revenue [Line Items]
Other revenues	30,775	22,478	84,953	65,810
Other [Member]
Disaggregation of Revenue [Line Items]
Other revenues	20,976	23,615	79,235	69,348
Other [Member]
Disaggregation of Revenue [Line Items]
Other revenues	91,419	48,973	263,978	145,785
Other [Member] | Revenue from contracts with customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	60,266	26,099	177,880	78,702
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	17,516	7,841	64,142	25,737
Other [Member] | Investment management [Member] | Revenue from contracts with customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	42,750	18,258	113,738	52,965
Other [Member] | Program services and other fronting arrangements [Member]
Disaggregation of Revenue [Line Items]
Other revenues	30,775	22,478	84,953	65,810
Other [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Other revenues	378	396	1,145	1,273
Markel Ventures [Member]
Disaggregation of Revenue [Line Items]
Other revenues	496,081	469,271	1,567,831	1,440,084
Markel Ventures [Member] | Revenue from contracts with customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	475,483	446,052	1,489,741	1,372,009
Markel Ventures [Member] | Products [Member] | Revenue from contracts with customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	378,421	350,981	1,198,124	1,095,461
Markel Ventures [Member] | Services [Member] | Revenue from contracts with customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	97,062	95,071	291,617	276,548
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 arrangements [Member]
Disaggregation of Revenue [Line Items]
Other revenues	0	0	0	0
Markel Ventures [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Other revenues	$ 20,598	$ 23,219	$ 78,090	$ 68,075