Recent Accounting Pronouncements (Narrative) (Details) $ in Millions	Dec. 31, 2015 USD ($)
Accounting Standards Update 2015-03 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Debt issuance costs reclassified from other assets to senior long-term debt and other debt	$ 2.2

Amount reclassified on the balance sheet from other assets to senior long-term debt and other debt due to a change in accounting principle. The balance sheet classification had no impact on net assets.

No definition available.

Name:	mkl_NewAccountingPronouncementChangeinAccountingPrincipleBalanceSheetReclassification
Namespace Prefix:	mkl_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant