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Derivatives Additional Collateral (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified | Mar. 31, 2015 | Dec. 31, 2014 | Mar. 31, 2014
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Credit risk on derivative instruments |  |  |
Collateral cash and investments | $ 33.5 wtm_CollateralCashAndInvestments | $ 33.2 wtm_CollateralCashAndInvestments | $ 36.5 wtm_CollateralCashAndInvestments
Cash |  |  |
Credit risk on derivative instruments |  |  |
Derivative collateral right to reclaim | 29.0 wtm_DerivativeCollateralRightToReclaim / us-gaap_InvestmentTypeAxis = us-gaap_CashAndCashEquivalentsMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 23.7 wtm_DerivativeCollateralRightToReclaim / us-gaap_InvestmentTypeAxis = us-gaap_CashAndCashEquivalentsMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 22.2 wtm_DerivativeCollateralRightToReclaim / us-gaap_InvestmentTypeAxis = us-gaap_CashAndCashEquivalentsMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Fixed maturity investments. |  |  |
Credit risk on derivative instruments |  |  |
Derivative collateral right to reclaim | $ 4.5 wtm_DerivativeCollateralRightToReclaim / us-gaap_InvestmentTypeAxis = us-gaap_FixedMaturitiesMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | $ 9.5 wtm_DerivativeCollateralRightToReclaim / us-gaap_InvestmentTypeAxis = us-gaap_FixedMaturitiesMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | $ 14.3 wtm_DerivativeCollateralRightToReclaim / us-gaap_InvestmentTypeAxis = us-gaap_FixedMaturitiesMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember

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

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- Definition Represents cash and short-term investments held and posted as collateral to reinsurance counterparties. + References No definition available. + Details
Name: | wtm_CollateralCashAndInvestments
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition The amount of collateral held against amounts due to counterparties for over the counter derivative instruments against net derivative instrument positions. + References No definition available. + Details
Name: | wtm_DerivativeCollateralRightToReclaim
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant