XML 65 R54.htm IDEA: XBRL DOCUMENT
v3.23.2

Unpaid Losses And Loss Adjustment Expenses (Narrative) (Details) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2023 | Jun. 30, 2022 | Jun. 30, 2023 | Jun. 30, 2022
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Incurred losses and loss adjustment expenses, current accident years | $ 1,255,767 | $ 1,085,215 | $ 2,500,098 | $ 2,153,910
Favorable (adverse) development on prior years' loss reserves |  |  | 139,208 | 122,728
Reinsurance recoverable for retroactive reinsurance transaction | (125,067) | 0 | (125,067) | 0
Insurance [Member] |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Incurred losses and loss adjustment expenses, current accident years | 1,079,450 | 919,283 | 2,156,996 | 1,805,520
Insurance [Member] | Professional liability, Property, Marine and Energy, and Workers Comp, Insurance segment [Member] |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Favorable (adverse) development on prior years' loss reserves |  |  | 169,600 |
Insurance [Member] | General Liability [Member] |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Favorable (adverse) development on prior years' loss reserves |  |  | (53,000) |
Insurance [Member] | Property, Marine and Energy, Workers Comp, and Programs, Insurance Segment [Member] |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Favorable (adverse) development on prior years' loss reserves |  |  |  | 123,300
Reinsurance [Member] |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Incurred losses and loss adjustment expenses, current accident years | 176,317 | $ 165,932 | 343,102 | $ 348,390
Reinsurance recoverable for retroactive reinsurance transaction | $ (125,100) |  | (125,100) |
Reinsurance [Member] | Professional liability and Property, Reinsurance segment [Member] |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Favorable (adverse) development on prior years' loss reserves |  |  | $ 20,900 |

X
- Definition Reinsurance Recoverable, Retroactive Reinsurance Transaction + References No definition available. + Details
Name: | mkl_ReinsuranceRecoverableRetroactiveReinsuranceTransaction
Namespace Prefix: | mkl_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount, after effects of reinsurance, of expense for claims incurred in the current reporting period and related claims settlement costs. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480081/944-40-50-3 + Details
Name: | us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersCurrentYearClaimsAndClaimsAdjustmentExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount, after effects of reinsurance, of expense (reversal of expense) for claims incurred in prior reporting periods and related claims settlement costs. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480081/944-40-50-3 + Details
Name: | us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=mkl_InsuranceMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_ProductOrServiceAxis=mkl_ProfessionalLiabilityPropertyMarineAndEnergyAndWorkersCompInsuranceSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_ProductOrServiceAxis=us-gaap_GeneralLiabilityMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_ProductOrServiceAxis=mkl_PropertyMarineAndEnergyWorkersCompAndProgramsInsuranceSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=mkl_ReinsuranceMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_ProductOrServiceAxis=mkl_ProfessionalLiabilityAndPropertyReinsuranceSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |