Markel Corporation (Parent Company Only) Financial Information (Schedule of Condensed Balance Sheets - Parenthetical) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Condensed Financial Statements, Captions [Line Items]
Fixed maturities, amortized cost	$ 9,448,840	$ 9,950,773
Equity securities, cost	3,266,735	2,971,856
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Fixed maturities, amortized cost	658,557	717,666
Equity securities, cost	$ 1,487,478	$ 1,117,363