Segment Reporting Disclosures (Reconciliation Of Segment Assets To The Company's Consolidated Balance Sheets (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Assets	$ 37,036,196	$ 33,306,263
Other [Member]
Segment Reporting Information [Line Items]
Assets	5,988,372	5,628,983
Segment assets [Member]
Segment Reporting Information [Line Items]
Assets	31,047,824	27,677,280
Segment assets [Member] | Investing [Member]
Segment Reporting Information [Line Items]
Assets	21,893,706	19,100,790
Segment assets [Member] | Underwriting [Member]
Segment Reporting Information [Line Items]
Assets	6,896,240	6,451,984
Segment assets [Member] | Markel Ventures [Member]
Segment Reporting Information [Line Items]
Assets	$ 2,257,878	$ 2,124,506