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Derivatives (Level 3 Derivative Instruments) (Details) - White Mountains Life Re
$ in Millions
12 Months Ended
Dec. 31, 2015
USD ($)
Discounted cash flows  
Quantitative information for Level 3 Fair Value Measurements Liabilities  
Fair value Liabilities $ (0.3)
Equity Index Options | Counterparty valuations [Member]  
Quantitative information for Level 3 Fair Value Measurements Liabilities  
Fair value Liabilities $ 0.8
Equity Index Options | Counterparty valuations [Member] | Minimum  
Quantitative information for Level 3 Fair Value Measurements Liabilities  
Adjustment to counterparty valuations 0.70%
Equity Index Options | Counterparty valuations [Member] | Maximum  
Quantitative information for Level 3 Fair Value Measurements Liabilities  
Adjustment to counterparty valuations (6.70%)
Equity Index Options | Counterparty valuations [Member] | Weighted Average  
Quantitative information for Level 3 Fair Value Measurements Liabilities  
Adjustment to counterparty valuations (2.50%)
Variable annuity benefit guarantee liabilities | Discounted cash flows | Minimum  
Quantitative information for Level 3 Fair Value Measurements Liabilities  
Mortality 0.00%
Variable annuity benefit guarantee liabilities | Discounted cash flows | Maximum  
Quantitative information for Level 3 Fair Value Measurements Liabilities  
Mortality 6.40%
Variable annuity benefit guarantee liabilities | Discounted cash flows | Weighted Average  
Quantitative information for Level 3 Fair Value Measurements Liabilities  
Mortality 1.00%
Foreign Exchange Option [Member] | Counterparty valuations [Member]  
Quantitative information for Level 3 Fair Value Measurements Liabilities  
Fair value Liabilities $ 1.9
Foreign Exchange Option [Member] | Counterparty valuations [Member] | Minimum  
Quantitative information for Level 3 Fair Value Measurements Liabilities  
Foreign exchange volatilities (0.50%)
Foreign Exchange Option [Member] | Counterparty valuations [Member] | Maximum  
Quantitative information for Level 3 Fair Value Measurements Liabilities  
Foreign exchange volatilities (7.30%)
Foreign Exchange Option [Member] | Counterparty valuations [Member] | Weighted Average  
Quantitative information for Level 3 Fair Value Measurements Liabilities  
Foreign exchange volatilities (5.90%)
Year One [Member] | Variable annuity benefit guarantee liabilities | Discounted cash flows | Minimum  
Quantitative information for Level 3 Fair Value Measurements Liabilities  
Surrenders 0.10%
Foreign exchange volatilities 11.50%
Index volatilities 25.00%
Year One [Member] | Variable annuity benefit guarantee liabilities | Discounted cash flows | Maximum  
Quantitative information for Level 3 Fair Value Measurements Liabilities  
Surrenders 40.00%
Foreign exchange volatilities 18.70%
Index volatilities 27.50%
Year One [Member] | Variable annuity benefit guarantee liabilities | Discounted cash flows | Weighted Average  
Quantitative information for Level 3 Fair Value Measurements Liabilities  
Surrenders 40.00%
Foreign exchange volatilities 13.10%
Index volatilities 26.40%