v2.3.0.15

Reinsurance (Narrative) (Details) (USD $) In Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Reinsurance Disclosures [Line Items]
Reinsurance recoverables	$ 42.9	$ 24.1	$ 136.7	$ 87.9
Catastrophe Losses [Member]
Reinsurance Disclosures [Line Items]
Reinsurance recoverables			38.4
Marine And Energy Large Losses [Member]
Reinsurance Disclosures [Line Items]
Reinsurance recoverables			26.4
Deepwater Horizon [Member]
Reinsurance Disclosures [Line Items]
Reinsurance recoverables				$ 43.2

X

- Definition

Reinsurance Disclosures [Line Items]

+ Details

Name:	mkl_ReinsuranceDisclosuresLineItems
Namespace Prefix:	mkl
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The reduction in the amount of the provision for the period for claims and benefits incurred on contract risks ceded to other insurers under reinsurance arrangements.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 113

-Paragraph 27, 113

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 20

-Section 50

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=6480758&loc=d3e6811-158387

+ Details

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