Variable Interest Entities (Narrative) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Markel Diversified Fund [Member]
Variable Interest Entity [Line Items]
Variable interest entity, consolidated, carrying amount, assets	$ 19.6	$ 28.6
Unconsolidated Markel CATCo Fund 2 [Member]
Variable Interest Entity [Line Items]
Variable interest entity, consolidated, carrying amount, investment in one of the unconsolidated funds	26.8	26.2
Unconsolidated Markel CATCo Funds [Member]
Variable Interest Entity [Line Items]
Net assets under management of MCIM for unconsolidated VIEs	$ 2,700.0	$ 3,400.0

The Company's investment in one of the unconsolidated Funds.

No definition available.

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- Definition

Net investment and insurance assets under management related to unconsolidated variable interest entities.

+ References

No definition available.

+ Details

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- Definition

The carrying amount of the consolidated Variable Interest Entity's assets included in the reporting entity's statement of financial position.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (bb)
-URI http://asc.fasb.org/extlink&oid=118582676&loc=d3e5710-111685

+ Details

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- 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

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- Details

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- Details

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- Details

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