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, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Document Period End Date			Jun. 30, 2019
Other revenues	$ 702,171	$ 628,193	$ 1,244,309	$ 1,067,625
Revenue From Contracts With Customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	645,094	581,432	1,131,872	978,560
Products [Member] | Revenue From Contracts With Customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	486,209	461,007	819,703	744,480
Services [Member] | Revenue From Contracts With Customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	128,789	103,007	241,181	199,373
Investment Management [Member] | Revenue From Contracts With Customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	30,096	17,418	70,988	34,707
Program Services and Other Fronting Arrangements [Member]
Disaggregation of Revenue [Line Items]
Other revenues	27,861	22,635	54,178	43,332
Other [Member]
Disaggregation of Revenue [Line Items]
Other revenues	29,216	24,126	58,259	45,733
Other [Member]
Disaggregation of Revenue [Line Items]
Other revenues	85,184	49,437	172,559	96,812
Other [Member] | Revenue From Contracts With Customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	56,977	26,390	117,614	52,603
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	26,881	8,972	46,626	17,896
Other [Member] | Investment Management [Member] | Revenue From Contracts With Customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	30,096	17,418	70,988	34,707
Other [Member] | Program Services and Other Fronting Arrangements [Member]
Disaggregation of Revenue [Line Items]
Other revenues	27,861	22,635	54,178	43,332
Other [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Other revenues	346	412	767	877
Markel Ventures [Member]
Disaggregation of Revenue [Line Items]
Other revenues	616,987	578,756	1,071,750	970,813
Markel Ventures [Member] | Revenue From Contracts With Customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	588,117	555,042	1,014,258	925,957
Markel Ventures [Member] | Products [Member] | Revenue From Contracts With Customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	486,209	461,007	819,703	744,480
Markel Ventures [Member] | Services [Member] | Revenue From Contracts With Customers [Member]
Disaggregation of Revenue [Line Items]
Other revenues	101,908	94,035	194,555	181,477
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	$ 28,870	$ 23,714	$ 57,492	$ 44,856