Segment Reporting Disclosures (Reconciliation Of Segment Assets To The Company's Consolidated Balance Sheets) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Assets	$ 37,473,815	$ 33,306,263	$ 32,805,016
Investing [Member]
Segment Reporting Information [Line Items]
Assets	22,129,633	19,100,790	20,317,160
Underwriting Operations [Member]
Segment Reporting Information [Line Items]
Assets	6,621,639	6,451,984	6,828,048
Markel Ventures [Member]
Segment Reporting Information [Line Items]
Assets	2,550,835	2,124,506	1,900,728
Other [Member]
Segment Reporting Information [Line Items]
Assets	6,171,708	5,628,983	3,759,080
Segment Assets [Member]
Segment Reporting Information [Line Items]
Assets	$ 31,302,107	$ 27,677,280	$ 29,045,936