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Derivatives (Collateral Provided to WM Life Re from Counterparties) (Details) (White Mountains Life Re [Member], USD $) In Millions, unless otherwise specified | Sep. 30, 2012 | Dec. 31, 2011
--- | --- | ---
Variable Annuity Reinsurance [Line Items] |  |
Derivative Collateral Obligation to Return | $ 85.6 | $ 83.0
Derivative Collateral Right to Reclaim | 63.3 | 73.2
Short-term investments |  |
Variable Annuity Reinsurance [Line Items] |  |
Derivative Collateral Obligation to Return | 63.3 | 73.2
Derivative Collateral Right to Reclaim | 14.6 | 0.6
Fixed maturity investments. |  |
Variable Annuity Reinsurance [Line Items] |  |
Derivative Collateral Obligation to Return | 0 | 0
Derivative Collateral Right to Reclaim | $ 121.2 | $ 31.2

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- Definition The amount of collateral that has been posted against amounts owed by counterparties to over the counter derivative instruments against net derivative instrument positions. + References No definition available. + Details
Name: | wtm_DerivativeCollateralObligationToReturn
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

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