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Derivatives (Value, Collateral (held) provided by WM Life Re and Net Exposure to Credit Losses on OTC Derivative Instruments) (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified | Dec. 31, 2012 | Dec. 31, 2011
--- | --- | ---
White Mountains Life Re |  |  |  |
Credit risk on derivative instruments |  |  |  |
OTC derivative instruments | $ 123.5 | [1] | $ 295.4 | [1]
Collateral held | (30.6) |  | (73.2) |
Collateral provided | 119.3 |  | 83.0 |
Net exposure on fair value of OTC instruments | 212.2 |  | 305.2 |
Derivative counterparty credit risk | $ (3.6) |  | $ (9.1) |
[1] | Value of OTC derivative instruments as of December 31, 2012 and 2011 excludes adjustments for counterparty credit risk of $(3.6) and $(9.1) included in fair value under GAAP.

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- Definition Fair values as of the balance sheet date of the net amount of all assets and liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 107 -Paragraph 10 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 133 -Paragraph 4, 17 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
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- Definition The amount of collateral that has been posted against amounts owed by counterparties to over the counter derivative instruments against net derivative instrument positions. + References No definition available. + Details
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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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- Definition Amount by which fair value of over the counter derivatives has been adjusted for counter party credit risk. + References No definition available. + Details
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