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Derivatives (Uncollateralized Amounts Due Under WM Life Re's OTC Derivative Contracts) (Details) - White Mountains Life Re $ in Thousands | Mar. 31, 2016 USD ($) Rating | Dec. 31, 2015 USD ($)
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Credit risk on derivative instruments |  |
Excess collateral provided - Financial Instruments | $ 13,100 | $ 20,100
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 | 13,100 | 20,100
Excess collateral provided to counterparty in cash | 7,900 | 5,800
Derivative, Collateral, Right to Reclaim Securities | 0 | 0
Counter-party collateral held by WMLife Re - Cash | 5,900 | 6,900
Net amount of exposure to counter-party | $ 15,100 | 19,000
S and P Credit Ratings | Rating | 23 |
Standard & Poor's, A Rating [Member] | Bank of America [Member] |  |
Credit risk on derivative instruments |  |
Excess collateral provided - Financial Instruments | $ 1,400 | 700
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 | 1,400 | 700
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 | 1,400 | 700
Standard & Poor's, A Rating [Member] | Citigroup - OTC |  |
Credit risk on derivative instruments |  |
Excess collateral provided - Financial Instruments | 5,000 | 9,900
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 | 5,000 | 9,900
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 | 500 | 1,400
Net amount of exposure to counter-party | 4,500 | 8,500
Standard & Poor's, A Rating [Member] | Citigroup Exchange Traded [Member] |  |
Credit risk on derivative instruments |  |
Excess collateral provided - Financial Instruments | 200 | 1,000
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 | 200 | 1,000
Excess collateral provided to counterparty in cash | 7,900 | 5,800
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 | 8,100 | 6,800
Standard & Poor's, A plus | JP Morgan |  |
Credit risk on derivative instruments |  |
Excess collateral provided - Financial Instruments | 6,500 | 8,500
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 | 6,500 | 8,500
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 | 5,400 | 5,500
Net amount of exposure to counter-party | $ 1,100 | $ 3,000

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