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Derivatives (Uncollateralized Amounts Due Under WM Life Re's OTC Derivative Contracts) (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified | Mar. 31, 2013 Rating | Dec. 31, 2012
--- | --- | ---
Credit risk on derivative instruments |  |  |  |
Collateral provided - Financial Instruments | $ 75.8 | [1] | $ 98.3 | [1]
Derivative, Collateral, Right to Reclaim Securities | 25.6 |  | 28.1 |
Collateral held - Financial Instruments | 0 |  | 0 |
Collateral provided - Cash | 89.5 |  | 126.3 |
Collateral held - Cash | 10.8 |  | 30.6 |
Net amount of exposure after effect of collateral held | 180.1 |  | 222.1 |
S and P Credit Ratings | 21 |  |  |
A | Bank of America |  |  |  |
Credit risk on derivative instruments |  |  |  |
Collateral provided - Financial Instruments | 60.0 |  | 78.5 |
Derivative, Collateral, Right to Reclaim Securities | 0 |  | 0 |
Collateral held - Financial Instruments | 0 |  | 0 |
Collateral provided - Cash | 0 |  | 0 |
Collateral held - Cash | 10.8 |  | 30.6 |
Net amount of exposure after effect of collateral held | 49.2 |  | 47.9 |
A | Royal Bank of Scotland |  |  |  |
Credit risk on derivative instruments |  |  |  |
Collateral provided - Financial Instruments | 26.4 |  | 33.6 |
Derivative, Collateral, Right to Reclaim Securities | 0 |  | 0 |
Collateral held - Financial Instruments | 0 |  | 0 |
Collateral provided - Cash | 0 |  | 0 |
Collateral held - Cash | 0 |  | 0 |
Net amount of exposure after effect of collateral held | 26.4 |  | 33.6 |
A | Citigroup - OTC |  |  |  |
Credit risk on derivative instruments |  |  |  |
Collateral provided - Financial Instruments | (5.3) |  | 19.9 |
Derivative, Collateral, Right to Reclaim Securities | 0 |  | 0 |
Collateral held - Financial Instruments | 0 |  | 0 |
Collateral provided - Cash | 25.3 |  | 30.8 |
Collateral held - Cash | 0 |  | 0 |
Net amount of exposure after effect of collateral held | 20.0 |  | 50.7 |
A | Citigroup - Exchange Traded |  |  |  |
Credit risk on derivative instruments |  |  |  |
Collateral provided - Financial Instruments | (8.7) |  | (21.5) |
Derivative, Collateral, Right to Reclaim Securities | 0 |  | 0 |
Collateral held - Financial Instruments | 0 |  | 0 |
Collateral provided - Cash | 24.7 |  | 35.0 |
Collateral held - Cash | 0 |  | 0 |
Net amount of exposure after effect of collateral held | 16.0 |  | 13.5 |
A | Barclays |  |  |  |
Credit risk on derivative instruments |  |  |  |
Collateral provided - Financial Instruments | 6.5 |  | 11.6 |
Derivative, Collateral, Right to Reclaim Securities | 0 |  | 0 |
Collateral held - Financial Instruments | 0 |  | 0 |
Collateral provided - Cash | 0 |  | 0 |
Collateral held - Cash | 0 |  | 0 |
Net amount of exposure after effect of collateral held | 6.5 |  | 11.6 |
A | JP Morgan |  |  |  |
Credit risk on derivative instruments |  |  |  |
Collateral provided - Financial Instruments | 3.1 |  | (22.8) |
Derivative, Collateral, Right to Reclaim Securities | 0 |  | 0 |
Collateral held - Financial Instruments | 0 |  | 0 |
Collateral provided - Cash | 31.3 |  | 55.6 |
Collateral held - Cash | 0 |  | 0 |
Net amount of exposure after effect of collateral held | 34.4 |  | 32.8 |
Standard & Poor's, BBB Plus Rating | Nomura |  |  |  |
Credit risk on derivative instruments |  |  |  |
Collateral provided - Financial Instruments | (6.2) |  | (0.9) |
Derivative, Collateral, Right to Reclaim Securities | 25.6 |  | 28.1 |
Collateral held - Financial Instruments | 0 |  | 0 |
Collateral provided - Cash | 8.2 |  | 1.7 |
Collateral held - Cash | 0 |  | 0 |
Net amount of exposure after effect of collateral held | 27.6 |  | 28.9 |
A- | Goldman Sachs |  |  |  |
Credit risk on derivative instruments |  |  |  |
Collateral provided - Financial Instruments | 0 |  | (0.1) |
Derivative, Collateral, Right to Reclaim Securities | 0 |  | 0 |
Collateral held - Financial Instruments | 0 |  | 0 |
Collateral provided - Cash | 0 |  | 3.2 |
Collateral held - Cash | 0 |  | 0 |
Net amount of exposure after effect of collateral held | $ 0 |  | $ 3.1 |
[1] | All derivative instruments held by WM Life Re are subject to master netting arrangements.

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- Definition The amount of the obligation to return cash collateral under master netting arrangements that have not been offset against net derivative instrument positions. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=7476318&loc=d3e41678-113959 + Details
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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 in accordance with the entity's accounting policy were offset against collateral under a master netting arrangement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=7476318&loc=d3e41678-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name FASB Staff Position (FSP) -Number FIN39-1 -Paragraph 10B -Subparagraph b -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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