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Summary of Significant Accounting Policies (Cash and Cash Equivalents) (Details)	12 Months Ended
	Dec. 31, 2024
Maximum [Member]
Cash and Cash Equivalents [Line Items]
Investment maturity period, cash and cash equivalents	90 days

X

- Definition

Investment maturity period, cash and cash equivalents investment type

+ References

No definition available.

+ Details

Name:	mkl_InvestmentMaturityPeriodCashAndCashEquivalents
Namespace Prefix:	mkl_
Data Type:	xbrli:durationItemType
Balance Type:	na
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_CashAndCashEquivalentsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: