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Derivatives Additional Collateral (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified | Sep. 30, 2014 | Dec. 31, 2013 | Sep. 30, 2013
--- | --- | --- | ---
Credit risk on derivative instruments |  |  |
Collateral cash and investments | $ 34.2 | $ 81.3 | $ 114.8
Cash |  |  |
Credit risk on derivative instruments |  |  |
Derivative collateral right to reclaim | 23.8 | 56.1 | 74.4
Short-term investments |  |  |
Credit risk on derivative instruments |  |  |
Derivative collateral right to reclaim | 0 | 2.0 | 15.8
Fixed maturity investments. |  |  |
Credit risk on derivative instruments |  |  |
Derivative collateral right to reclaim | $ 10.4 | $ 23.2 | $ 24.6

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