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Derivatives (Details) In Billions, unless otherwise specified | Dec. 31, 2012 USD ($) | Dec. 31, 2012 JPY (¥)
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Derivative Instruments and Hedging Activities Disclosure [Abstract] |  |
Total guarantee value | $ 2.7 | ¥ 230.0
Collective account values of underlying variable annuities as a percentage of the guarantee value | 87.00% | 87.00%

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- Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Represents the collective account values of underlying variable annuities as a percentage of the guarantee value of the associated variable annuity reinsurance liability. + References No definition available. + Details
Name: | wtm_CollectiveAccountValueAsPercentageOfGuaranteedValue
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Total notional account value guaranteed with certain variable annuities. + References No definition available. + Details
Name: | wtm_VariableAnnuityNotionalGuaranteeValue
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant