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Commitments and Contingencies (Legal Contingencies) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended |
--- | --- | ---
Dec. 31, 2014 | Jun. 30, 2007
Legal Contingency [Abstract] |  |
Period for which company has indemnified Allstate for breaches of representations and warranties (in months) | 18 months |
Period for which the entity has agreed not to solicit certain employees of transferred subsidiaries (in years) | 3 years |
Percentage of positive or negative development in the loss reserves of the Transferred Subsidiaries retained | 90.00%wtm_PercentageOfFluctuationInLossReservesOfTransferredSubsidiariesRetained |
Tribune Company Litigation |  |
Legal Contingency [Abstract] |  |
Proceeds in connection with Tribune leveraged buyout |  | $ 39.0 wtm_ProceedsFromCommonStock / us-gaap_LossContingenciesByNatureOfContingencyAxis = wtm_TribuneCompanyLitigationMember

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- Details
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- Definition Percentage of favorable or unfavorable loss reserve development in the Transferred Subsidiaries retained by the entity. + References No definition available. + Details
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- Definition Represents the period from the transaction closing date for which the entity has agreed not to solicit certain employees of the transferred subsidiaries as per the covenants of sale agreement for the sale of subsidiaries. + References No definition available. + Details
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- Definition Represents the period from the transaction closing date for which the entity has provided indemnity to the counterparty of the agreement against the breaches of representations and warranties specified in the sale agreement for the sale of subsidiaries. + References No definition available. + Details
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- Definition Proceeds from common stock tendered in connection with a leveraged buyout. + References No definition available. + Details
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