Contingencies (Narrative) (Details) - Thomas Yeransian v. Markel Corporation [Member] $ in Millions	Sep. 30, 2017 USD ($)
Damages [Member]
Loss Contingencies [Line Items]
Loss contingency, estimate of possible loss	$ 47.3
Interest [Member]
Loss Contingencies [Line Items]
Loss contingency, estimate of possible loss	11.1
Default Interest [Member]
Loss Contingencies [Line Items]
Loss contingency, estimate of possible loss	$ 9.7

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-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://www.xbrl.org/2003/role/presentationRef
-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

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