Derivatives (Level 3 Derivative Instruments) (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2014
Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Liabilities
Fair value Liabilities	$ 11.7
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)	5.90%
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	15.4
Foreign Exchange Options | Counterparty valuations | Minimum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Foreign exchange volatilities (as a percent)	(1.20%)
Foreign Exchange Options | Counterparty valuations | Maximum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Foreign exchange volatilities (as a percent)	8.00%
Foreign Exchange Options | Counterparty valuations | Weighted Average
Quantitative information for Level 3 Fair Value Measurements Liabilities
Foreign exchange volatilities (as a percent)	2.60%
Equity Index Options | Counterparty valuations
Quantitative information for Level 3 Fair Value Measurements Liabilities
Fair value Liabilities	$ 20.5
Equity Index Options | Counterparty valuations | Minimum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Expected equity dividends (as a percent)	0.50%
Equity Index Options | Counterparty valuations | Maximum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Expected equity dividends (as a percent)	9.70%
Equity Index Options | Counterparty valuations | Weighted Average
Quantitative information for Level 3 Fair Value Measurements Liabilities
Expected equity dividends (as a percent)	3.20%
Year One | Variable annuity benefit guarantee liabilities | Discounted cash flows | Minimum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Surrenders (as a percent)	0.20%
Foreign exchange volatilities (as a percent)	10.30%
Index volatilities (as a percent)	18.30%
Year One | Variable annuity benefit guarantee liabilities | Discounted cash flows | Maximum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Surrenders (as a percent)	32.00%
Foreign exchange volatilities (as a percent)	13.00%
Index volatilities (as a percent)	21.00%
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)	26.00%
Foreign exchange volatilities (as a percent)	10.90%
Index volatilities (as a percent)	20.30%
Year Two | Variable annuity benefit guarantee liabilities | Discounted cash flows | Minimum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Surrenders (as a percent)	0.10%
Foreign exchange volatilities (as a percent)	10.60%
Index volatilities (as a percent)	18.20%
Year Two | Variable annuity benefit guarantee liabilities | Discounted cash flows | Maximum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Surrenders (as a percent)	14.00%
Foreign exchange volatilities (as a percent)	12.60%
Index volatilities (as a percent)	21.10%
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)	7.20%
Foreign exchange volatilities (as a percent)	11.40%
Index volatilities (as a percent)	20.30%