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Summary of Significant Accounting Policies (Investment Securities) (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
security
Dec. 31, 2013
Accounting Policies [Abstract]    
Percentage of investments recorded at fair value 93.00%wtm_PercentageOfInvestmentsRecordedAtFairValue 95.00%wtm_PercentageOfInvestmentsRecordedAtFairValue
Minimum percentage of variation expected 5.00%wtm_MinimumPercentageOfVariationFromExpectedPriceRequiredToTreatPricesOfInvestmentsProvidedByPricingServicesAsOutliers  
Minimum variation expected $ 1.0wtm_MinimumVariationFromExpectedPriceRequiredToTreatPricesOfInvestmentsProvidedByPricingServicesAsOutliers  
Securities outside variance threshold 21wtm_SecuritiesOutsideVarianceThresholds