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Derivatives (Details) - USD ($) $ in Millions | Mar. 31, 2017 | Dec. 31, 2015
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Notional amount | $ 299.9 |
Collective account values of underlying variable annuities as a percentage of the guarantee value |  | 113.00%

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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
Name: | invest_DerivativeNotionalAmount
Namespace Prefix: | invest_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | instant

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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