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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

X

- 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

X

- Details

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