Reflects the estimated amount of loss from the specified contingency as of the balance sheet date.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14435-108349

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 9
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14557-108349

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=115931803&loc=d3e12021-110248

Name:	us-gaap_LossContingencyEstimateOfPossibleLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Contingencies (Narrative) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Feb. 21, 2019	Dec. 31, 2018
Loss Contingencies [Line Items]
Percentage of Investors that Redeemed Shares	73.00%
Percentage of Net Assets under Management Redeemed	91.00%
Markel Ventures [Member] \| Industrial Products [Member]
Loss Contingencies [Line Items]
Loss contingency, estimate of possible loss	$ 26.4		$ 33.5
Anthony Belisle v. Markel CATCo Investment Management Ltd and Markel Corp. [Member]
Loss Contingencies [Line Items]
Asserted claims		$ 66.0
Alissa Fredricks v. Markel CATCo Investment Management Ltd. and Markel Corp. [Member]
Loss Contingencies [Line Items]
Asserted claims		$ 7.5