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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 | Jun. 30, 2013 Rating | Dec. 31, 2012
--- | --- | ---
Credit risk on derivative instruments |  |  |  |
Excess collateral provided - Financial Instruments | $ 141.3 | [1] | $ 98.3 | [1]
Collertal provided to counterparty in cash | 9.1 |  | 45.3 |
Collateral provided to counterparty in financial instruments | 0.60 |  | 0 |
Net amount of exposure after effect of collateral | 151.0 |  | 143.6 |
Excess collateral provided to counterparty in cash | 13.6 |  | 81.1 |
Derivative, Collateral, Right to Reclaim Securities | 23.7 |  | 28.0 |
Counter-party collateral held by WMLife Re - Cash | 30.4 |  | 30.6 |
Net amount of exposure to counter-party | 157.9 |  | 222.1 |
S and P Credit Ratings | 21 |  |  |
A | Bank of America [Member] |  |  |  |
Credit risk on derivative instruments |  |  |  |
Excess collateral provided - Financial Instruments | 52.1 |  | 78.5 |
Collertal provided to counterparty in cash | 0 |  | 0 |
Collateral provided to counterparty in financial instruments | 0 |  | 0 |
Net amount of exposure after effect of collateral | 52.1 |  | 78.5 |
Excess collateral provided to counterparty in cash | 0 |  | 0 |
Derivative, Collateral, Right to Reclaim Securities | 0 |  | 0 |
Counter-party collateral held by WMLife Re - Cash | 6.3 |  | 30.6 |
Net amount of exposure to counter-party | 45.8 |  | 47.9 |
A | Barclays |  |  |  |
Credit risk on derivative instruments |  |  |  |
Excess collateral provided - Financial Instruments | 4.6 |  |  |
Collertal provided to counterparty in cash | 0 |  |  |
Collateral provided to counterparty in financial instruments | 0 |  |  |
Net amount of exposure after effect of collateral | 4.6 |  |  |
Excess collateral provided to counterparty in cash | 0 |  |  |
Derivative, Collateral, Right to Reclaim Securities | 0 |  |  |
Counter-party collateral held by WMLife Re - Cash | 0 |  |  |
Net amount of exposure to counter-party | 4.6 |  |  |
A | JP Morgan |  |  |  |
Credit risk on derivative instruments |  |  |  |
Excess collateral provided - Financial Instruments | 34.1 |  |  |
Collertal provided to counterparty in cash | 0 |  |  |
Collateral provided to counterparty in financial instruments | 0 |  |  |
Net amount of exposure after effect of collateral | 34.1 |  |  |
Excess collateral provided to counterparty in cash | 0 |  |  |
Derivative, Collateral, Right to Reclaim Securities | 0 |  |  |
Counter-party collateral held by WMLife Re - Cash | 5.9 |  |  |
Net amount of exposure to counter-party | 28.2 |  |  |
A | Royal Bank of Scotland |  |  |  |
Credit risk on derivative instruments |  |  |  |
Excess collateral provided - Financial Instruments | 21.7 |  | 33.6 |
Collertal provided to counterparty in cash | 0 |  | 0 |
Collateral provided to counterparty in financial instruments | 0 |  | 0 |
Net amount of exposure after effect of collateral | 21.7 |  | 33.6 |
Excess collateral provided to counterparty in cash | 0 |  | 0 |
Derivative, Collateral, Right to Reclaim Securities | 0 |  | 0 |
Counter-party collateral held by WMLife Re - Cash | 0 |  | 0 |
Net amount of exposure to counter-party | 21.7 |  | 33.6 |
A | Citigroup - OTC |  |  |  |
Credit risk on derivative instruments |  |  |  |
Excess collateral provided - Financial Instruments | 38.5 |  | 19.9 |
Collertal provided to counterparty in cash | 0 |  | 0 |
Collateral provided to counterparty in financial instruments | 0 |  | 0 |
Net amount of exposure after effect of collateral | 38.5 |  | 19.9 |
Excess collateral provided to counterparty in cash | 0 |  | 30.8 |
Derivative, Collateral, Right to Reclaim Securities | 0 |  | 0 |
Counter-party collateral held by WMLife Re - Cash | 18.2 |  | 0 |
Net amount of exposure to counter-party | 20.3 |  | 50.7 |
A | Citigroup Exchange Traded [Member] |  |  |  |
Credit risk on derivative instruments |  |  |  |
Excess collateral provided - Financial Instruments | (3.4) |  | (21.5) |
Collertal provided to counterparty in cash | 3.4 |  | 21.5 |
Collateral provided to counterparty in financial instruments | 0 |  | 0 |
Net amount of exposure after effect of collateral | 0 |  | 0 |
Excess collateral provided to counterparty in cash | 13.6 |  | 13.6 |
Derivative, Collateral, Right to Reclaim Securities | 0 |  | 0 |
Counter-party collateral held by WMLife Re - Cash | 0 |  | 0 |
Net amount of exposure to counter-party | 13.6 |  | 13.6 |
A | Barclays |  |  |  |
Credit risk on derivative instruments |  |  |  |
Excess collateral provided - Financial Instruments |  |  | 11.6 |
Collertal provided to counterparty in cash |  |  | 0 |
Collateral provided to counterparty in financial instruments |  |  | 0 |
Net amount of exposure after effect of collateral |  |  | 11.6 |
Excess collateral provided to counterparty in cash |  |  | 0 |
Derivative, Collateral, Right to Reclaim Securities |  |  | 0 |
Counter-party collateral held by WMLife Re - Cash |  |  | 0 |
Net amount of exposure to counter-party |  |  | 11.6 |
A | JP Morgan |  |  |  |
Credit risk on derivative instruments |  |  |  |
Excess collateral provided - Financial Instruments |  |  | (22.8) |
Collertal provided to counterparty in cash |  |  | 22.8 |
Collateral provided to counterparty in financial instruments |  |  | 0 |
Net amount of exposure after effect of collateral |  |  | 0 |
Excess collateral provided to counterparty in cash |  |  | 32.8 |
Derivative, Collateral, Right to Reclaim Securities |  |  | 0 |
Counter-party collateral held by WMLife Re - Cash |  |  | 0 |
Net amount of exposure to counter-party |  |  | 32.8 |
Standard & Poor's, BBB+ Rating [Member] | Nomura [Member] |  |  |  |
Credit risk on derivative instruments |  |  |  |
Excess collateral provided - Financial Instruments | (6.3) |  | (0.9) |
Collertal provided to counterparty in cash | 5.7 |  | 0.9 |
Collateral provided to counterparty in financial instruments | 0.60 |  | 0 |
Net amount of exposure after effect of collateral | 0 |  | 0 |
Excess collateral provided to counterparty in cash | 0 |  | 0.8 |
Derivative, Collateral, Right to Reclaim Securities | 23.7 |  | 28.0 |
Counter-party collateral held by WMLife Re - Cash | 0 |  | 0 |
Net amount of exposure to counter-party | 23.7 |  | 28.8 |
A- | Goldman Sachs |  |  |  |
Credit risk on derivative instruments |  |  |  |
Excess collateral provided - Financial Instruments | 0 |  | (0.1) |
Collertal provided to counterparty in cash | 0 |  | 0.1 |
Collateral provided to counterparty in financial instruments | 0 |  | 0 |
Net amount of exposure after effect of collateral | 0 |  | 0 |
Excess collateral provided to counterparty in cash | 0 |  | 3.1 |
Derivative, Collateral, Right to Reclaim Securities | 0 |  | 0 |
Counter-party collateral held by WMLife Re - Cash | 0 |  | 0 |
Net amount of exposure to counter-party | $ 0 |  | $ 3.1 |
[1] | All derivative instruments held by WM Life Re are subject to master netting arrangements.

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