Derivatives (Level 3 Derivative Instruments) (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014
Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Liabilities
Fair value Liabilities	0.7
Equity Index Options | Counterparty valuations [Member]
Quantitative information for Level 3 Fair Value Measurements Liabilities
Fair value Liabilities	11.7
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	6.50%
Equity Index Options | Counterparty valuations [Member] | Weighted Average
Quantitative information for Level 3 Fair Value Measurements Liabilities
Adjustment to counterparty valuations	2.00%
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	5.90%
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	7.2
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	11.60%
Foreign Exchange Option [Member] | Counterparty valuations [Member] | Weighted Average
Quantitative information for Level 3 Fair Value Measurements Liabilities
Foreign exchange volatilities	3.60%
Year Two [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	11.70%
Index volatilities	19.40%
Year Two [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	13.80%
Index volatilities	23.10%
Year Two [Member] | Variable annuity benefit guarantee liabilities | Discounted cash flows | Weighted Average
Quantitative information for Level 3 Fair Value Measurements Liabilities
Surrenders	32.10%
Foreign exchange volatilities	12.60%
Index volatilities	21.80%
Year One [Member] | Variable annuity benefit guarantee liabilities | Discounted cash flows | Minimum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Surrenders	0.20%
Foreign exchange volatilities	12.60%
Index volatilities	24.00%
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	15.10%
Index volatilities	26.60%
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	13.30%
Index volatilities	25.60%