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Summary of Significant Accounting Policies (Investment Securities) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2012 security
Accounting Policies [Abstract] |
Percentage of investments recorded at fair value | 95.00%
Minimum percentage of variation expected | 5.00%
Minimum variation expected | $ 1
Securities outside variance threshold | 15

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- Details
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- Definition Represents the minimum percentage of variation identified during sales backtesting of prices of investments provided by pricing services from the historical sales price, required for treating the prices provided by pricing services as outliers. + References No definition available. + Details
Name: | wtm_MinimumPercentageOfVariationFromExpectedPriceRequiredToTreatPricesOfInvestmentsProvidedByPricingServicesAsOutliers
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- Definition Represents the minimum dollar amount of variation identified during sales backtesting of prices of investments provided by pricing services from the historical sales price, required for treating the prices provided by pricing services as outliers. + References No definition available. + Details
Name: | wtm_MinimumVariationFromExpectedPriceRequiredToTreatPricesOfInvestmentsProvidedByPricingServicesAsOutliers
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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- Definition Represents the percentage of the total investment portfolio recorded at fair value based upon quoted market prices or other observable inputs. + References No definition available. + Details
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Data Type: | num:percentItemType
Balance Type: | na
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- Definition Represents the number of securities that fall outside White Mountains' variance thresholds in the period, thereby triggering a challenge with the pricing service. + References No definition available. + Details
Name: | wtm_SecuritiesOutsideVarianceThresholds
Namespace Prefix: | wtm_
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Balance Type: | na
Period Type: | duration