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Derivatives Additional Collateral (Details) (White Mountains Life Re [Member], USD $) In Millions, unless otherwise specified | Sep. 30, 2012 | Dec. 31, 2011 | Sep. 30, 2011
--- | --- | --- | ---
Credit risk on derivative instruments |  |  |
Derivative Collateral Right to Reclaim | $ 63.3 | $ 73.2 |
Collateral Cash and Investments | 477.5 | 485.3 | 419.7
Cash and Cash Equivalents [Member] |  |  |
Credit risk on derivative instruments |  |  |
Derivative Collateral Right to Reclaim | 341.7 | 453.5 |
Short-term investments |  |  |
Credit risk on derivative instruments |  |  |
Derivative Collateral Right to Reclaim | 14.6 | 0.6 |
Fixed maturity investments. |  |  |
Credit risk on derivative instruments |  |  |
Derivative Collateral Right to Reclaim | $ 121.2 | $ 31.2 |

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- Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
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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
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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