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Derivatives (Uncollateralized Amounts Due Under WM Life Re's OTC Derivative Contracts) (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2012 | Dec. 31, 2011
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White Mountains Life Re |  |  |
Credit risk on derivative instruments |  |  |
Net exposure on fair value of OTC instruments | $ 212.2 |  | $ 305.2
Citigroup | White Mountains Life Re |  |  |
Credit risk on derivative instruments |  |  |
Net exposure on fair value of OTC instruments | 54.9 | [1] |
JP Morgan | White Mountains Life Re |  |  |
Credit risk on derivative instruments |  |  |
Net exposure on fair value of OTC instruments | 30.9 | [1] |
Nomura | White Mountains Life Re |  |  |
Credit risk on derivative instruments |  |  |
Net exposure on fair value of OTC instruments | 26.9 | [1] |
Barclays | White Mountains Life Re |  |  |
Credit risk on derivative instruments |  |  |
Net exposure on fair value of OTC instruments | 12.3 |  |
Goldman Sachs | White Mountains Life Re |  |  |
Credit risk on derivative instruments |  |  |
Net exposure on fair value of OTC instruments | 3.1 |  |
Bank of America [Member] | White Mountains Life Re |  |  |
Credit risk on derivative instruments |  |  |
Net exposure on fair value of OTC instruments | 49.6 |  |
Royal Bank of Scotland [Member] | White Mountains Life Re |  |  |
Credit risk on derivative instruments |  |  |
Net exposure on fair value of OTC instruments | 34.5 |  |
Forward contracts |  |  |
Credit risk on derivative instruments |  |  |
Net exposure on fair value of OTC instruments | 31.2 |  |
S&P Credit Ratings | 21 |  |
Forward contracts | JP Morgan |  |  |
Credit risk on derivative instruments |  |  |
Net exposure on fair value of OTC instruments | 1.9 |  |
Forward contracts | Goldman Sachs |  |  |
Credit risk on derivative instruments |  |  |
Net exposure on fair value of OTC instruments | $ 0.4 |  |
[1] | Collateral provided (held) calculated under master netting agreement.

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- Definition The net amount as of the balance sheet date of the fair value of derivative assets and derivative liabilities that were not covered by collateral. + References No definition available. + Details
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