Derivatives (Level 3 Derivative Instruments) (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015
Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Liabilities
Fair value Liabilities	1.1
Variable annuity benefit guarantee liabilities | Discounted cash flows | Minimum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Mortality (as a percent)	0.00%
Variable annuity benefit guarantee liabilities | Discounted cash flows | Maximum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Mortality (as a percent)	6.40%
Variable annuity benefit guarantee liabilities | Discounted cash flows | Weighted Average
Quantitative information for Level 3 Fair Value Measurements Liabilities
Mortality (as a percent)	1.00%
Foreign Exchange Option [Member] | Counterparty valuations
Quantitative information for Level 3 Fair Value Measurements Liabilities
Fair value Liabilities	11.1
Foreign Exchange Option [Member] | Counterparty valuations | Minimum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Foreign exchange volatilities (as a percent)	0.50%
Foreign Exchange Option [Member] | Counterparty valuations | Maximum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Foreign exchange volatilities (as a percent)	11.60%
Foreign Exchange Option [Member] | Counterparty valuations | Weighted Average
Quantitative information for Level 3 Fair Value Measurements Liabilities
Foreign exchange volatilities (as a percent)	5.90%
Equity Index Options | Counterparty valuations
Quantitative information for Level 3 Fair Value Measurements Liabilities
Fair value Liabilities	5.9
Equity Index Options | Counterparty valuations | Minimum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Expected equity dividends (as a percent)	(0.70%)
Equity Index Options | Counterparty valuations | Maximum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Expected equity dividends (as a percent)	6.50%
Equity Index Options | Counterparty valuations | Weighted Average
Quantitative information for Level 3 Fair Value Measurements Liabilities
Expected equity dividends (as a percent)	2.10%
Year One | Variable annuity benefit guarantee liabilities | Discounted cash flows | Minimum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Surrenders (as a percent)	0.10%
Foreign exchange volatilities (as a percent)	10.90%
Index volatilities (as a percent)	24.80%
Year One | Variable annuity benefit guarantee liabilities | Discounted cash flows | Maximum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Surrenders (as a percent)	40.00%
Foreign exchange volatilities (as a percent)	16.20%
Index volatilities (as a percent)	27.20%
Year One | Variable annuity benefit guarantee liabilities | Discounted cash flows | Weighted Average
Quantitative information for Level 3 Fair Value Measurements Liabilities
Surrenders (as a percent)	40.00%
Foreign exchange volatilities (as a percent)	13.10%
Index volatilities (as a percent)	26.70%
Year Two | Variable annuity benefit guarantee liabilities | Discounted cash flows | Minimum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Surrenders (as a percent)	0.20%
Foreign exchange volatilities (as a percent)	10.90%
Index volatilities (as a percent)	21.20%
Year Two | Variable annuity benefit guarantee liabilities | Discounted cash flows | Maximum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Surrenders (as a percent)	40.00%
Foreign exchange volatilities (as a percent)	14.10%
Index volatilities (as a percent)	23.90%
Year Two | Variable annuity benefit guarantee liabilities | Discounted cash flows | Weighted Average
Quantitative information for Level 3 Fair Value Measurements Liabilities
Surrenders (as a percent)	32.20%
Foreign exchange volatilities (as a percent)	12.50%
Index volatilities (as a percent)	22.90%