v2.3.0.15

Fair Value Measurements (Narrative) (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Fair Value Measurements [Abstract]
Fair value level 1 to level 2 transfers amount	$ 0	$ 0	$ 0	$ 0
Fair value level 2 to level 1 transfers amount	0	0	0	0
Assets fair value disclosure nonrecurring	0	0	0	0
Liabilities fair value disclosure nonrecurring	$ 0	$ 0	$ 0	$ 0

X

- Definition

This element represents the aggregate of the liabilities reported on the balance sheet at period end measured at fair value on a nonrecurring basis by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements.

+ Details

Name:	mkl_LiabilitiesFairValueDisclosureNonrecurring
Namespace Prefix:	mkl
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

This element represents the aggregate of the assets reported on the balance sheet at period end measured at fair value on a nonrecurring basis by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements.

+ Details

Name:	us-gaap_AssetsFairValueDisclosureNonrecurring
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Details

Name:	us-gaap_FairValueDisclosuresAbstract
Namespace Prefix:	us-gaap
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The amount of the transfers of assets or liabilities measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (bb)

-URI http://asc.fasb.org/extlink&oid=6925170&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueLevel1ToLevel2TransfersAmount
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

The amount of the transfers of assets or liabilities measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (bb)

-URI http://asc.fasb.org/extlink&oid=6925170&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueLevel2ToLevel1TransfersAmount
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration