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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, 2014 Rating | Dec. 31, 2013
--- | --- | ---
Credit risk on derivative instruments |  |
Excess collateral provided - Financial Instruments | $ 89.6 | $ 69.2
Collertal provided to counterparty in cash | 0 | 0
Collateral provided to counterparty in financial instruments | 0 | 0.40
Net amount of exposure after effect of collateral | 89.6 | 69.6
Excess collateral provided to counterparty in cash | 20.4 | 44.1
Derivative, Collateral, Right to Reclaim Securities | 14.3 | 22.8
Counter-party collateral held by WMLife Re - Cash | 3.8 | 0.8
Net amount of exposure to counter-party | 120.5 | 135.7
S and P Credit Ratings | 21 |
Standard & Poor's, A Rating | Bank of America [Member] |  |
Credit risk on derivative instruments |  |
Excess collateral provided - Financial Instruments | 29.1 | 27.2
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 | 29.1 | 27.2
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 | 29.1 | 27.2
Standard & Poor's, A Rating | Barclays |  |
Credit risk on derivative instruments |  |
Excess collateral provided - Financial Instruments | 1.2 |
Collertal provided to counterparty in cash | 0 |
Collateral provided to counterparty in financial instruments | 0 |
Net amount of exposure after effect of collateral | 1.2 |
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 | 1.2 |
Standard & Poor's, A Rating | Royal Bank of Scotland |  |
Credit risk on derivative instruments |  |
Excess collateral provided - Financial Instruments |  | 11.3
Collertal provided to counterparty in cash |  | 0
Collateral provided to counterparty in financial instruments |  | 0
Net amount of exposure after effect of collateral |  | 11.3
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 |  | 11.3
Standard & Poor's, A Rating | Citigroup - OTC |  |
Credit risk on derivative instruments |  |
Excess collateral provided - Financial Instruments | 24.2 | 19.4
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 | 24.2 | 19.4
Excess collateral provided to counterparty in cash | 0 | 2.3
Derivative, Collateral, Right to Reclaim Securities | 0 | 0
Counter-party collateral held by WMLife Re - Cash | 3.8 | 0
Net amount of exposure to counter-party | 20.4 | 21.7
Standard & Poor's, A Rating | Citigroup Exchange Traded [Member] |  |
Credit risk on derivative instruments |  |
Excess collateral provided - Financial Instruments | 0.4 | 1.2
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 | 0.4 | 1.2
Excess collateral provided to counterparty in cash | 10.9 | 19.8
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 | 11.3 | 21.0
Standard & Poor's, A plus | Barclays |  |
Credit risk on derivative instruments |  |
Excess collateral provided - Financial Instruments |  | 1.4
Collertal provided to counterparty in cash |  | 0
Collateral provided to counterparty in financial instruments |  | 0
Net amount of exposure after effect of collateral |  | 1.4
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 |  | 1.4
Standard & Poor's, A plus | JP Morgan |  |
Credit risk on derivative instruments |  |
Excess collateral provided - Financial Instruments | 22.6 | 9.1
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 | 22.6 | 9.1
Excess collateral provided to counterparty in cash | 4.7 | 22.0
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 | 27.3 | 31.1
Standard & Poor's, BBB+ Rating [Member] | Nomura [Member] |  |
Credit risk on derivative instruments |  |
Excess collateral provided - Financial Instruments | 0.7 | (0.4)
Collertal provided to counterparty in cash | 0 | 0
Collateral provided to counterparty in financial instruments | 0 | 0.40
Net amount of exposure after effect of collateral | 0.7 | 0
Excess collateral provided to counterparty in cash | 4.8 | 0
Derivative, Collateral, Right to Reclaim Securities | 14.3 | 22.8
Counter-party collateral held by WMLife Re - Cash | 0 | 0.8
Net amount of exposure to counter-party | 19.8 | 22.0
Standard & Poor's, A- Rating | Royal Bank of Scotland |  |
Credit risk on derivative instruments |  |
Excess collateral provided - Financial Instruments | 11.4 |
Collertal provided to counterparty in cash | 0 |
Collateral provided to counterparty in financial instruments | 0 |
Net amount of exposure after effect of collateral | 11.4 |
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 | $ 11.4 |

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