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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   $ 39.0wtm_ProceedsFromCommonStock
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= wtm_TribuneCompanyLitigationMember