Acquisitions (Calculation Of Total Purchase Price) (Details) - Alterra Capital Holdings [Member] $ / shares in Units, $ in Thousands	May. 01, 2013 USD ($) $ / shares shares
Business Acquisition [Line Items]
Exchange ratio per the Merger Agreement	0.04315
Markel share issuance to Alterra shareholders	4,300,000
Incentive award ratio per the Merger Agreement	0.06252
Multiplied by Markel's weighted average stock price on April 30, 2013 | $ / shares	$ 529.59	[1]
Markel share and restricted stock issuance consideration, net of taxes | $	$ 2,267,648
Alterra common shares outstanding as of the Acquisition Date that received cash consideration	96,433,000
Multiplied by cash price per share component per the Merger Agreement | $ / shares	$ 10.00
Markel cash consideration | $	$ 964,330
Fair value of Markel warrant issuance to Alterra warrant holders as of the Acquisition Date | $	73,685
Fair value of Markel stock option issuance to Alterra stock option holders as of the Acquisition Date, net of taxes | $	12,335
Fair value of partially vested Markel restricted stock unit issuance as of the Acquisition Date, net of taxes | $	6,867
Unrecognized compensation on unvested restricted stock and restricted stock units | $	(20,572)
Total acquisition consideration | $	$ 3,304,293
Common Stock [Member]
Business Acquisition [Line Items]
Shares of Alterra stock outstanding as of the Acquisition Date	96,433,000
Markel share issuance to Alterra shareholders	4,161,000
Restricted Stock [Member]
Business Acquisition [Line Items]
Shares of Alterra stock outstanding as of the Acquisition Date	2,239,000
Markel share issuance to Alterra shareholders	140,000

[1] The fair value of the shares issued by the Company was calculated as the weighted average price of the Company's stock on April 30, 2013, the day preceding the Acquisition Date.