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Recent Developments Related to COVID-19 Assets (Narrative) (Details) $ in Billions | Jun. 30, 2020 USD ($)
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Assets [Abstract] |
Goodwill and intangible assets | $ 4.5

X
- Definition Total of goodwill and intangible assets + References No definition available. + Details
Name: | mkl_GoodwillandIntangibleAssets
Namespace Prefix: | mkl_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- References No definition available. + Details
Name: | us-gaap_AssetsAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration