Segment Reporting Disclosures (Reconciliation Of Segment Assets To The Company's Consolidated Balance Sheets (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Assets	$ 36,421,584	$ 33,306,263
Other [Member]
Segment Reporting Information [Line Items]
Assets	5,711,950	5,628,983
Segment Assets [Member]
Segment Reporting Information [Line Items]
Assets	30,709,634	27,677,280
Segment Assets [Member] | Investing [Member]
Segment Reporting Information [Line Items]
Assets	21,214,083	19,100,790
Segment Assets [Member] | Underwriting [Member]
Segment Reporting Information [Line Items]
Assets	7,186,643	6,451,984
Segment Assets [Member] | Markel Ventures [Member]
Segment Reporting Information [Line Items]
Assets	$ 2,308,908	$ 2,124,506