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Derivatives (Details) In Billions, unless otherwise specified | Sep. 30, 2012 USD ($) | Sep. 30, 2012 JPY (¥) | Dec. 31, 2011 USD ($) | Dec. 31, 2011 JPY (¥)
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Derivatives Disclosure [Abstract] |  |  |  |
Total guarantee value | $ 3.0 | ¥ 230.5 | $ 3.0 | ¥ 233.7
Collective account values of underlying variable annuities as a percentage of the guarantee value | 80.00% | 80.00% | 78.00% | 78.00%

X
- 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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- Details
Name: | wtm_DerivativesDisclosureAbstract
Namespace Prefix: | wtm_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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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