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Recent Developments Related to COVID-19 Recent Developments Related to COVID-19 (Narrative) (Details) $ in Millions	6 Months Ended
	Jun. 30, 2020 USD ($)
COVID-19 Pandemic [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net losses and loss adjustment expenses, net of assumed reinstatement premiums	$ 325.0

X

- Definition

Amount, after effects of reinsurance and assumed reinstatement premiums, of expense related to provisions for policy benefits and costs incurred for property and casualty insurance contracts

+ References

No definition available.

+ Details

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Namespace Prefix:	mkl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

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

X

- Details

Name:	us-gaap_NoncatastrophicEventAxis=mkl_COVID19PandemicMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: