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Summary of Significant Accounting Policies (Details 2) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2011 security Y | Dec. 31, 2010
Significant Accounting Policies |  |
Percentage of the investment portfolio recorded at fair value based upon quoted market prices or other observable inputs | 95.00% | 93.00%
Maturity period to classify an investment as short-term (in years) | 1 |
Minimum percentage of variation from the expected price required to consider prices provided by the pricing services as outliers | 5.00% |
Minimum amount of variation from expected prices required to consider prices provided by the pricing services as outliers | $ 1 |
Number of securities that fell outside White Mountains' variance threshold | 4 |

X
- Details
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- Definition Represents the maturity period to classify an investment as short-term. + References No definition available. + Details
Name: | wtm_MaturityPeriodFromPurchaseDateToClassifyInvestmentAsShortTerm
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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
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X
- 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
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Data Type: | xbrli:monetaryItemType
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Period Type: | instant

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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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- 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_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration