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Recent Developments Related to COVID-19 Recent Developments Related to COVID-19 (Narrative) (Details) $ in Millions | 6 Months Ended
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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_
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Balance Type: | debit
Period Type: | duration

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

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