v3.2.0.727

Senior Long-Term Debt And Other Debt (Narrative) (Details) - Senior Credit Facility [Member] $ in Millions	May. 19, 2015 USD ($)
Line of Credit Facility [Line Items]
Previous line of credit facility, borrowing capacity	$ 650
Credit facility, maximum capacity	$ 550

X

- Definition

Maximum capacity under the facility for borrowings and letters of credit without consideration of any current restrictions on the amount that could be borrowed/issued or the amounts currently outstanding/issued under the facility.

+ References

No definition available.

+ Details

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

X

- Definition

Maximum borrowing capacity under the previous credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

No definition available.

+ Details

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

X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_LineOfCreditFacilityLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	us-gaap_CreditFacilityAxis=mkl_SeniorCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: