XML 51 R41.htm IDEA: XBRL DOCUMENT
v3.24.3

Investments (Summary Of Gross Unrealized Investment Losses By Length Of Time That Securities Have Continuously Been In An Unrealized Loss Position) (Details) - USD ($) $ in Thousands | Sep. 30, 2024 | Dec. 31, 2023
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale securities, estimated fair value, less than 12 months | $ 624,978 | $ 1,467,268
Available-for-sale securities, gross unrealized holding losses, less than 12 months | (1,794) | (10,821)
Available-for-sale securities, estimated fair value, 12 months or longer | 7,875,442 | 8,515,061
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (436,044) | (650,461)
Available-for-sale securities, estimated fair value | 8,500,420 | 9,982,329
Available-for-sale securities, gross unrealized holding losses | (437,838) | (661,282)
Short-term investments, estimated fair value, less than 12 months | 327,973 | 52,601
Short-term investments, gross unrealized holding losses, less than 12 months | (90) | (393)
Short-term investments, estimated fair value, 12 months or longer | 0 | 0
Short-term investments, gross unrealized holding losses, 12 months or longer | 0 | 0
Short-term investments, estimated fair value | 327,973 | 52,601
Short-term investments, gross unrealized holding losses | (90) | (393)
U.S. Treasury securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale securities, estimated fair value, less than 12 months | 52,587 | 317,027
Available-for-sale securities, gross unrealized holding losses, less than 12 months | (85) | (2,147)
Available-for-sale securities, estimated fair value, 12 months or longer | 1,420,909 | 1,507,784
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (31,809) | (60,257)
Available-for-sale securities, estimated fair value | 1,473,496 | 1,824,811
Available-for-sale securities, gross unrealized holding losses | (31,894) | (62,404)
U.S. government-sponsored enterprises [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale securities, estimated fair value, less than 12 months | 18,881 | 145,143
Available-for-sale securities, gross unrealized holding losses, less than 12 months | (584) | (2,134)
Available-for-sale securities, estimated fair value, 12 months or longer | 723,075 | 723,537
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (64,868) | (87,770)
Available-for-sale securities, estimated fair value | 741,956 | 868,680
Available-for-sale securities, gross unrealized holding losses | (65,452) | (89,904)
Obligations of states, municipalities and political subdivisions [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale securities, estimated fair value, less than 12 months | 153,371 | 679,124
Available-for-sale securities, gross unrealized holding losses, less than 12 months | (518) | (3,881)
Available-for-sale securities, estimated fair value, 12 months or longer | 2,249,754 | 2,332,281
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (128,810) | (177,697)
Available-for-sale securities, estimated fair value | 2,403,125 | 3,011,405
Available-for-sale securities, gross unrealized holding losses | (129,328) | (181,578)
Foreign governments, agencies and supranationals [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale securities, estimated fair value, less than 12 months | 16,767 | 83,396
Available-for-sale securities, gross unrealized holding losses, less than 12 months | (57) | (395)
Available-for-sale securities, estimated fair value, 12 months or longer | 1,316,288 | 1,492,912
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (103,587) | (141,613)
Available-for-sale securities, estimated fair value | 1,333,055 | 1,576,308
Available-for-sale securities, gross unrealized holding losses | (103,644) | (142,008)
Agency mortgage-backed securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale securities, estimated fair value, less than 12 months | 53,196 | 189,977
Available-for-sale securities, gross unrealized holding losses, less than 12 months | (412) | (1,871)
Available-for-sale securities, estimated fair value, 12 months or longer | 1,818,419 | 1,889,272
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (85,793) | (133,740)
Available-for-sale securities, estimated fair value | 1,871,615 | 2,079,249
Available-for-sale securities, gross unrealized holding losses | (86,205) | (135,611)
Non-agency mortgage-backed securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale securities, estimated fair value, less than 12 months | 0 | 0
Available-for-sale securities, gross unrealized holding losses, less than 12 months | 0 | 0
Available-for-sale securities, estimated fair value, 12 months or longer | 119,224 | 333,070
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (3,416) | (22,603)
Available-for-sale securities, estimated fair value | 119,224 | 333,070
Available-for-sale securities, gross unrealized holding losses | (3,416) | (22,603)
Corporate and university bonds [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale securities, estimated fair value, less than 12 months | 2,203 | 0
Available-for-sale securities, gross unrealized holding losses, less than 12 months | (48) | 0
Available-for-sale securities, estimated fair value, 12 months or longer | 227,773 | 236,205
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (17,761) | (26,781)
Available-for-sale securities, estimated fair value | 229,976 | 236,205
Available-for-sale securities, gross unrealized holding losses | (17,809) | (26,781)
Total fixed maturity securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale securities, estimated fair value, less than 12 months | 297,005 | 1,414,667
Available-for-sale securities, gross unrealized holding losses, less than 12 months | (1,704) | (10,428)
Available-for-sale securities, estimated fair value, 12 months or longer | 7,875,442 | 8,515,061
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (436,044) | (650,461)
Available-for-sale securities, estimated fair value | 8,172,447 | 9,929,728
Available-for-sale securities, gross unrealized holding losses | $ (437,748) | $ (660,889)

X
- Definition Amount of short-term investment measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset. + References No definition available. + Details
Name: | mkl_ShorttermInvestmentsAvailableforsaleContinuousUnrealizedLossPosition12MonthsorLonger
Namespace Prefix: | mkl_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of accumulated unrealized loss on investment in short-term investment measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without allowance for credit loss. Includes beneficial interest in securitized financial asset. + References No definition available. + Details
Name: | mkl_ShorttermInvestmentsAvailableforsaleContinuousUnrealizedLossPosition12MonthsorLongerAccumulatedLoss
Namespace Prefix: | mkl_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of short-term investment measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset + References No definition available. + Details
Name: | mkl_ShorttermInvestmentsAvailableforsaleContinuousUnrealizedLossPositionLessthan12Months
Namespace Prefix: | mkl_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of accumulated unrealized loss on short-term investment measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset. + References No definition available. + Details
Name: | mkl_ShorttermInvestmentsAvailableforsaleContinuousUnrealizedLossPositionLessthan12MonthsAccumulatedLoss
Namespace Prefix: | mkl_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of accumulated unrealized loss on investment in short-term investment measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset. + References No definition available. + Details
Name: | mkl_ShorttermInvestmentsAvailableforsaleUnrealizedLossPositionAccumulatedLoss
Namespace Prefix: | mkl_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of short-term investment measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position without allowance for credit loss. + References No definition available. + Details
Name: | mkl_ShorttermInvestmentsAvailableforsaleUnrealizedLossPositionTotal
Namespace Prefix: | mkl_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 55 -Paragraph 8 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479081/326-30-55-8 Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 7 -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-7 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479106/326-30-50-5 + Details
Name: | us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without allowance for credit loss. Includes beneficial interest in securitized financial asset. + References Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 7 -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-7 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479106/326-30-50-5 + Details
Name: | us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 55 -Paragraph 8 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479081/326-30-55-8 Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 7 -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-7 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479106/326-30-50-5 + Details
Name: | us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset. + References Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 7 -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-7 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479106/326-30-50-5 + Details
Name: | us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position without allowance for credit loss. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 55 -Paragraph 8 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479081/326-30-55-8 Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Subparagraph (a)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-6 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 4 -Subparagraph (a)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479106/326-30-50-4 + Details
Name: | us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPosition
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset. + References Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Subparagraph (a)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-6 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 4 -Subparagraph (a)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479106/326-30-50-4 + Details
Name: | us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss
Namespace Prefix: | us-gaap_
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 Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-2 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-2 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (aa) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-2 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (aaa) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-2 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-2 Reference 6: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-2 Reference 7: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (d) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-2 Reference 8: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-3 Reference 9: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-3 Reference 10: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-3 Reference 11: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-3 Reference 12: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (d) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-3 + Details
Name: | us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_USTreasurySecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_ForeignGovernmentDebtSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=mkl_AgencyMortgageBackedSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=mkl_NonAgencyMortgageBackedSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_AllOtherCorporateBondsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_FixedMaturitiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |