XML 86 R61.htm IDEA: XBRL DOCUMENT
v3.25.0.1

Segment Reporting Disclosures (Summary of Top Five Independent Brokers Concentration) (Details) - Top Five Independent Brokers | 12 Months Ended
--- | ---
Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022
Insurance [Member] |  |  |
Summary of Top Five Independent Brokers Concentration [Line Items] |  |  |
Percentage of gross premiums written | 32.00% | 33.00% | 34.00%
Reinsurance [Member] |  |  |
Summary of Top Five Independent Brokers Concentration [Line Items] |  |  |
Percentage of gross premiums written | 90.00% | 95.00% | 99.00%
Underwriting Operations [Member] |  |  |
Summary of Top Five Independent Brokers Concentration [Line Items] |  |  |
Percentage of gross premiums written | 38.00% | 37.00% | 40.00%

X
- Definition Percentage of gross premiums written + References No definition available. + Details
Name: | mkl_PercentageOfGrossPremiumsWritten
Namespace Prefix: | mkl_
Data Type: | dtr-types1:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Summary of Top Five Independent Brokers Concentration [Line Items] + References No definition available. + Details
Name: | mkl_SummaryOfTopFiveIndependentBrokersConcentrationLineItems
Namespace Prefix: | mkl_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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

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