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Summary of Significant Accounting Policies (Cash and Cash Equivalents) (Details) | 12 Months Ended
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Dec. 31, 2023
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
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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: |