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Derivatives (Uncollateralized Amounts Due Under WM Life Re's OTC Derivative Contracts) (Details) (White Mountains Life Re [Member], USD $) In Millions, unless otherwise specified | Sep. 30, 2012 Rating | Dec. 31, 2011
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Credit risk on derivative instruments |  |  |
Net Exposure On Fair Value Of Over The Counter Derivative Instruments | $ 273.7 |  | $ 305.2
S&P Credit Ratings | 22 |  |
Citigroup [Member] |  |  |
Credit risk on derivative instruments |  |  |
Net Exposure On Fair Value Of Over The Counter Derivative Instruments | 63.4 | [1] |
Royal Bank of Scotland [Member] |  |  |
Credit risk on derivative instruments |  |  |
Net Exposure On Fair Value Of Over The Counter Derivative Instruments | 58.5 |  |
Bank of America [Member] |  |  |
Credit risk on derivative instruments |  |  |
Net Exposure On Fair Value Of Over The Counter Derivative Instruments | 50.3 |  |
J P Morgan Chase [Member] |  |  |
Credit risk on derivative instruments |  |  |
Net Exposure On Fair Value Of Over The Counter Derivative Instruments | 40.8 | [1] |
Nomura [Member] |  |  |
Credit risk on derivative instruments |  |  |
Net Exposure On Fair Value Of Over The Counter Derivative Instruments | 35.9 | [1] |
Barclays [Member] |  |  |
Credit risk on derivative instruments |  |  |
Net Exposure On Fair Value Of Over The Counter Derivative Instruments | 20.7 |  |
Goldman Sachs [Member] |  |  |
Credit risk on derivative instruments |  |  |
Net Exposure On Fair Value Of Over The Counter Derivative Instruments | $ 4.1 | [1] |
[1] | (2) 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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