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Life and Annuity Benefits Life and Annuity Benefits (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Jun. 30, 2020 | Jun. 30, 2019 | Jun. 30, 2020 | Jun. 30, 2019 | Dec. 31, 2019
Liability for Future Policy Benefit, before Reinsurance [Abstract] |  |  |  |  |
Increase in life and annuity benefits | $ 22.7 | $ 36.0 | $ 35.6 | $ 61.8 |
Cumulative adjustment to life and annuity benefits | $ 87.0 |  | $ 87.0 |  | $ 51.4

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- Definition The adjustment to life and annuity benefits during the period attributable to the change in unrealized gains on the underlying investment portfolio during the period. + References No definition available. + Details
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- Definition The cumulative adjustment to life and annuity benefits attributable to unrealized gains on the underlying investment portfolio. + References No definition available. + Details
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Namespace Prefix: | mkl_
Data Type: | xbrli:monetaryItemType
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- References No definition available. + Details
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Namespace Prefix: | us-gaap_
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