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Subsequent Event (Details) (Subsequent Sale, Esurance Disposal, USD $) | 1 Months Ended |
--- | --- | ---
Oct. 31, 2011 | Oct. 07, 2011
Subsequent Sale | Esurance Disposal |  |
Subsequent Event |  |
Consideration received on sale of business | $ 1,010,000,000 |
Consideration received in excess of tangible book value |  | 700,000,000
Estimated pro forma tangible book value at closing of the legal entities sold |  | $ 310,000,000

X
- Definition This element represents the cash inflow during the period from the sale of a component of the entity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 15, 16 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Investing Activities -URI http://asc.fasb.org/extlink&oid=6516133 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 12 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3179-108585 + Details
Name: | us-gaap_ProceedsFromDivestitureOfBusinessesNetOfCashDivested
Namespace Prefix: | us-gaap
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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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. + Details
Name: | us-gaap_SubsequentEventLineItems
Namespace Prefix: | us-gaap
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Consideration in excess of tangible book value for the sale of a business. + Details
Name: | wtm_ConsiderationInExcessOfTangibleBookValueForTheSaleOfABusiness
Namespace Prefix: | wtm
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition For the disposal group, including a component of the entity (discontinued operation), it represents the tangible book value of the businesses sold by the entity. + Details
Name: | wtm_DisposalGroupIncludingDiscontinuedOperationBookValue
Namespace Prefix: | wtm
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant