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v3.23.3

Products, Services and Other Revenues (Schedule Of Receivables And Customer Deposits Related To Contracts With Customers) (Details) - USD ($) $ in Millions | Sep. 30, 2023 | Dec. 31, 2022
--- | --- | ---
Contracts with customers [Member] |  |
Receivables [Line Items] |  |
Trade and Other Receivables, Net | $ 683.1 | $ 624.1

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- Definition [Line Items] for Other Revenue [Table] + References No definition available. + Details
Name: | mkl_OtherRevenueLineItems
Namespace Prefix: | mkl_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Gross carrying amount as of the balance sheet date of all trade and other types of receivables due from other persons or entities. + References No definition available. + Details
Name: | mkl_TradeAndOtherReceivables
Namespace Prefix: | mkl_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Details
Name: | us-gaap_ConcentrationRiskByBenchmarkAxis=mkl_ContractsWithCustomersMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |