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Derivatives (Uncollateralized Amounts Due Under WM Life Re's OTC Derivative Contracts) (Details) - White Mountains Life Re $ in Thousands | Dec. 31, 2015 USD ($) Rating | Dec. 31, 2014 USD ($)
--- | --- | ---
Variable Annuity Reinsurance [Line Items] |  |
Net amounts recognized in Other Assets | $ 20,100 | $ 56,400
Credit risk on derivative instruments |  |
Net exposure on fair value of OTC instruments | $ 19,000 | 77,200
S&P Credit Ratings | Rating | 21 |
Derivative, Collateral, Right to Reclaim Cash | $ 0 | 3,500
Derivative, Collateral, Right to Reclaim Securities | 0 | 0
Derivative Asset, Fair Value, Amount Offset Against Collateral | 20,100 | 59,900
Excess collateral provided to counterparty in cash | 5,800 | 17,700
Excess collateral providedto counterparty in financial instruments | 0 | 9,500
Derivative, Collateral, Obligation to Return Cash | 6,900 | 9,900
A | Citigroup OTC [Member] |  |
Variable Annuity Reinsurance [Line Items] |  |
Net amounts recognized in Other Assets | 9,900 | 22,200
Credit risk on derivative instruments |  |
Net exposure on fair value of OTC instruments | 8,500 | 21,100
Derivative, Collateral, Right to Reclaim Cash | 0 | 0
Derivative, Collateral, Right to Reclaim Securities | 0 | 0
Derivative Asset, Fair Value, Amount Offset Against Collateral | 9,900 | 22,200
Excess collateral provided to counterparty in cash | 0 | 0
Excess collateral providedto counterparty in financial instruments | 0 | 0
Derivative, Collateral, Obligation to Return Cash | 1,400 | 1,100
A | Royal Bank of Scotland [Member] |  |
Variable Annuity Reinsurance [Line Items] |  |
Net amounts recognized in Other Assets |  | 4,000
Credit risk on derivative instruments |  |
Net exposure on fair value of OTC instruments |  | 4,000
Derivative, Collateral, Right to Reclaim Cash |  | 0
Derivative, Collateral, Right to Reclaim Securities |  | 0
Derivative Asset, Fair Value, Amount Offset Against Collateral |  | 4,000
Excess collateral provided to counterparty in cash |  | 0
Excess collateral providedto counterparty in financial instruments |  | 0
Derivative, Collateral, Obligation to Return Cash |  | 0
A | Barclays [Member] |  |
Variable Annuity Reinsurance [Line Items] |  |
Net amounts recognized in Other Assets |  | 100
Credit risk on derivative instruments |  |
Net exposure on fair value of OTC instruments |  | 100
Derivative, Collateral, Right to Reclaim Cash |  | 0
Derivative, Collateral, Right to Reclaim Securities |  | 0
Derivative Asset, Fair Value, Amount Offset Against Collateral |  | 100
Excess collateral provided to counterparty in cash |  | 0
Excess collateral providedto counterparty in financial instruments |  | 0
Derivative, Collateral, Obligation to Return Cash |  | 0
A | Bank of America [Member] |  |
Variable Annuity Reinsurance [Line Items] |  |
Net amounts recognized in Other Assets | 700 | 5,600
Credit risk on derivative instruments |  |
Net exposure on fair value of OTC instruments | 700 | 5,600
Derivative, Collateral, Right to Reclaim Cash | 0 | 0
Derivative, Collateral, Right to Reclaim Securities | 0 | 0
Derivative Asset, Fair Value, Amount Offset Against Collateral | 700 | 5,600
Excess collateral provided to counterparty in cash | 0 | 0
Excess collateral providedto counterparty in financial instruments | 0 | 0
Derivative, Collateral, Obligation to Return Cash | 0 | 0
A | Citigroup Exchange Traded [Member] |  |
Variable Annuity Reinsurance [Line Items] |  |
Net amounts recognized in Other Assets | 1,000 | 3,700
Credit risk on derivative instruments |  |
Net exposure on fair value of OTC instruments | 6,800 | 19,700
Derivative, Collateral, Right to Reclaim Cash | 0 | 0
Derivative, Collateral, Right to Reclaim Securities | 0 | 0
Derivative Asset, Fair Value, Amount Offset Against Collateral | 1,000 | 3,700
Excess collateral provided to counterparty in cash | 5,800 | 16,000
Excess collateral providedto counterparty in financial instruments | 0 | 0
Derivative, Collateral, Obligation to Return Cash | 0 | 0
BBB plus | Nomura [Member] |  |
Variable Annuity Reinsurance [Line Items] |  |
Net amounts recognized in Other Assets |  | (3,500)
Credit risk on derivative instruments |  |
Net exposure on fair value of OTC instruments |  | 11,200
Derivative, Collateral, Right to Reclaim Cash |  | 3,500
Derivative, Collateral, Right to Reclaim Securities |  | 0
Derivative Asset, Fair Value, Amount Offset Against Collateral |  | 0
Excess collateral provided to counterparty in cash |  | 1,700
Excess collateral providedto counterparty in financial instruments |  | 9,500
Derivative, Collateral, Obligation to Return Cash |  | 0
Standard & Poor's, A+ Rating [Member] | J P Morgan Chase [Member] |  |
Variable Annuity Reinsurance [Line Items] |  |
Net amounts recognized in Other Assets | 8,500 | 24,300
Credit risk on derivative instruments |  |
Net exposure on fair value of OTC instruments | 3,000 | 15,500
Derivative, Collateral, Right to Reclaim Cash | 0 | 0
Derivative, Collateral, Right to Reclaim Securities | 0 | 0
Derivative Asset, Fair Value, Amount Offset Against Collateral | 8,500 | 24,300
Excess collateral provided to counterparty in cash | 0 | 0
Excess collateral providedto counterparty in financial instruments | 0 | 0
Derivative, Collateral, Obligation to Return Cash | $ 5,500 | $ 8,800

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