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Summary of Significant Accounting Policies (Insurance and Reinsurance Operations) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2012 | Dec. 31, 2011
Discount rate on workers compensation loss and loss adjustment reserves | 3.50% | 4.50%
Discount amount on workers compensation loss and loss adjustment reserves | $ 4.6 | $ 108.3
OneBeacon Runoff |  |
Discount amount on workers compensation loss and loss adjustment reserves | $ 77.9 |

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- Definition Represents the amount by which workers compensation loss and LAE reserves are discounted. + References No definition available. + Details
Name: | wtm_WorkersCompensationLossAndLossAdjustmentReservesDiscountAmount
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Represents the average discount rate used to discount certain of the long-term workers compensation loss and LAE reserves, which is determined based on the various assumptions including consideration of when the claims will be settled. + References No definition available. + Details
Name: | wtm_WorkersCompensationLossAndLossAdjustmentReservesDiscountRate
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration