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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

X

- Definition

Capital expenditure

+ References

No definition available.

+ Details

Name:	mkl_CapitalExpenditure
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_SegmentReportingInformationLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- 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: