Segment Reporting Disclosures (Reconciliation Of Segment Assets To The Company's Consolidated Balance Sheets) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Assets	$ 33,306,263	$ 32,805,016	$ 25,875,299
Investing [Member]
Segment Reporting Information [Line Items]
Assets	19,100,790	20,317,160	19,029,584
Underwriting Operations [Member]
Segment Reporting Information [Line Items]
Assets	6,451,984	6,828,048	5,397,696
Markel Ventures [Member]
Segment Reporting Information [Line Items]
Assets	2,124,506	1,900,728	1,206,223
Other [Member]
Segment Reporting Information [Line Items]
Assets	5,628,983	3,759,080	241,796
Segment Assets [Member]
Segment Reporting Information [Line Items]
Assets	$ 27,677,280	$ 29,045,936	$ 25,633,503