Segment Reporting Disclosures (Narrative) (Details) - segment	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenue, Major Customer [Line Items]
Number of reportable ongoing underwriting segments	2
Underwriting Operations [Member]
Revenue, Major Customer [Line Items]
Percentage of gross premiums written	100.00%	100.00%	100.00%
Markel Ventures [Member]
Revenue, Major Customer [Line Items]
Percentage of Revenue Attributable to U.S.	90.00%	88.00%	85.00%
Top Three Independent Brokers [Member] | Underwriting Operations [Member]
Revenue, Major Customer [Line Items]
Percentage of gross premiums written	28.00%	25.00%	27.00%
Top Three Independent Brokers [Member] | Insurance [Member]
Revenue, Major Customer [Line Items]
Percentage of gross premiums written	17.00%	13.00%	14.00%
Top Three Independent Brokers [Member] | Reinsurance [Member]
Revenue, Major Customer [Line Items]
Percentage of gross premiums written	82.00%	76.00%	78.00%