Products, Services and Other Revenues (Schedule Of Revenues From Contracts With Customers By Type) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Other revenues	$ 542,138	$ 439,432
Revenue From Contracts With Customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	486,779	397,128
Markel Ventures [Member]
Disaggregation of Revenue [Line Items]
Other revenues	454,763	392,057
Markel Ventures [Member] | Revenue From Contracts With Customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	426,141	370,915
Other [Member]
Disaggregation of Revenue [Line Items]
Other revenues	87,375	47,375
Other [Member] | Revenue From Contracts With Customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	60,638	26,213
Products [Member] | Revenue From Contracts With Customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	333,494	283,473
Products [Member] | Markel Ventures [Member] | Revenue From Contracts With Customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	333,494	283,473
Products [Member] | Other [Member] | Revenue From Contracts With Customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	0	0
Services [Member] | Revenue From Contracts With Customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	112,392	96,366
Services [Member] | Markel Ventures [Member] | Revenue From Contracts With Customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	92,647	87,442
Services [Member] | Other [Member] | Revenue From Contracts With Customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	19,745	8,924
Investment Management [Member] | Revenue From Contracts With Customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	40,893	17,289
Investment Management [Member] | Markel Ventures [Member] | Revenue From Contracts With Customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	0	0
Investment Management [Member] | Other [Member] | Revenue From Contracts With Customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	40,893	17,289
Program Services [Member]
Disaggregation of Revenue [Line Items]
Other revenues	24,109	20,697
Program Services [Member] | Markel Ventures [Member]
Disaggregation of Revenue [Line Items]
Other revenues	0	0
Program Services [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Other revenues	24,109	20,697
Other [Member]
Disaggregation of Revenue [Line Items]
Other revenues	31,250	21,607
Other [Member] | Markel Ventures [Member]
Disaggregation of Revenue [Line Items]
Other revenues	28,622	21,142
Other [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Other revenues	$ 2,628	$ 465