Markel Corporation (Parent Company Only) Financial Information (Schedule of Condensed Balance Sheets - Parenthetical) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Condensed Financial Statements, Captions [Line Items]
Fixed maturities, amortized cost	$ 9,950,773	$ 9,551,153
Equity securities, available-for-sale, cost		2,667,661
Equity securities, cost	2,971,856
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Fixed maturities, amortized cost	717,666	533,183
Equity securities, available-for-sale, cost		$ 402,694
Equity securities, cost	$ 1,117,363