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Segment Reporting Disclosures (Summary of Additional Segment Measures) (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2025 | Sep. 30, 2024
Industrial |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Capital expenditure | $ 24,267 | $ 24,962 | $ 58,136 | $ 105,095
Consumer and Other |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Capital expenditure | $ 14,848 | $ 9,516 | $ 41,471 | $ 33,097

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- Definition Capital expenditure + References No definition available. + Details
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Namespace Prefix: | mkl_
Data Type: | xbrli:monetaryItemType
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
Name: | us-gaap_SegmentReportingInformationLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=mkl_IndustrialMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=mkl_ConsumerAndOtherMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |