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Derivatives (Additional Collateral) (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified | Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010
--- | --- | --- | ---
Credit risk on derivative instruments |  |  |
Derivative Collateral Right to Reclaim | $ 30.6 | $ 73.2 |
Collateral Cash and Investments | 393.6 | 485.3 |
Cash |  |  |
Credit risk on derivative instruments |  |  |
Derivative Collateral Right to Reclaim | 249.8 | 453.5 | 326.0
Short-term investments |  |  |
Credit risk on derivative instruments |  |  |
Derivative Collateral Right to Reclaim | 5.1 | 0.6 |
Fixed maturity investments |  |  |
Credit risk on derivative instruments |  |  |
Derivative Collateral Right to Reclaim | $ 138.7 | $ 31.2 |

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