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Derivatives (Additional Collateral) (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
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Credit risk on derivative instruments |  |  |
Collateral Cash and Investments | $ 33.2 wtm_CollateralCashAndInvestments | $ 81.3 wtm_CollateralCashAndInvestments | $ 393.6 wtm_CollateralCashAndInvestments
Cash |  |  |
Credit risk on derivative instruments |  |  |
Derivative Collateral Right to Reclaim | 23.7 wtm_DerivativeCollateralRightToReclaim / us-gaap_InvestmentTypeAxis = us-gaap_CashAndCashEquivalentsMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 56.1 wtm_DerivativeCollateralRightToReclaim / us-gaap_InvestmentTypeAxis = us-gaap_CashAndCashEquivalentsMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember |
Short-term investments |  |  |
Credit risk on derivative instruments |  |  |
Derivative Collateral Right to Reclaim | 0 wtm_DerivativeCollateralRightToReclaim / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 2.0 wtm_DerivativeCollateralRightToReclaim / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember |
Fixed maturity investments |  |  |
Credit risk on derivative instruments |  |  |
Derivative Collateral Right to Reclaim | $ 9.5 wtm_DerivativeCollateralRightToReclaim / us-gaap_InvestmentTypeAxis = us-gaap_FixedMaturitiesMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | $ 23.2 wtm_DerivativeCollateralRightToReclaim / us-gaap_InvestmentTypeAxis = us-gaap_FixedMaturitiesMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember |

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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 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
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Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant