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Derivatives (Details) In Billions, unless otherwise specified | Mar. 31, 2015 USD ($) | Mar. 31, 2015 JPY (¥) | Dec. 31, 2014 USD ($) | Dec. 31, 2014 JPY (¥)
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Derivative Instruments and Hedging Activities Disclosure [Abstract] |  |  |  |
Notional amount | $ 1.0 invest_DerivativeNotionalAmount | ¥ 117.8 invest_DerivativeNotionalAmount | $ 1.1 invest_DerivativeNotionalAmount | ¥ 134.2 invest_DerivativeNotionalAmount
Collective account values of underlying variable annuities as a percentage of the guarantee value | 113.00%wtm_CollectiveAccountValueAsPercentageOfGuaranteedValue | 113.00%wtm_CollectiveAccountValueAsPercentageOfGuaranteedValue | 113.00%wtm_CollectiveAccountValueAsPercentageOfGuaranteedValue | 113.00%wtm_CollectiveAccountValueAsPercentageOfGuaranteedValue

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- Definition Aggregate notional amount specified by the derivative(s). Expressed as an absolute value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | instant

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- Details
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Balance Type: | na
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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
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Namespace Prefix: | wtm_
Data Type: | num:percentItemType
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