Segment Reporting Disclosures (Reconciliation Of Segment Assets To The Company's Consolidated Balance Sheets (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Assets	$ 34,736,726	$ 33,306,263
Other [Member]
Segment Reporting Information [Line Items]
Assets	5,706,174	5,628,983
Segment Assets [Member]
Segment Reporting Information [Line Items]
Assets	29,030,552	27,677,280
Segment Assets [Member] | Investing [Member]
Segment Reporting Information [Line Items]
Assets	19,914,906	19,100,790
Segment Assets [Member] | Underwriting [Member]
Segment Reporting Information [Line Items]
Assets	6,869,350	6,451,984
Segment Assets [Member] | Markel Ventures [Member]
Segment Reporting Information [Line Items]
Assets	$ 2,246,296	$ 2,124,506