Derivatives (Level 3 Derivative Instruments) (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2013
Variable annuity benefit guarantee liabilities
Quantitative information for Level 3 Fair Value Measurements Liabilities
Fair value Liabilities	$ 194.1
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)	5.30%
Variable annuity benefit guarantee liabilities | Discounted cash flows | Weighted Average
Quantitative information for Level 3 Fair Value Measurements Liabilities
Mortality (as a percent)	0.90%
Foreign Exchange Options
Quantitative information for Level 3 Fair Value Measurements Liabilities
Fair value Liabilities	69.3
Foreign Exchange Options | Counterparty valuations [Member] | Minimum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Foreign exchange volatilities (as a percent)	0.10%
Foreign Exchange Options | Counterparty valuations [Member] | Maximum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Foreign exchange volatilities (as a percent)	3.20%
Foreign Exchange Options | Counterparty valuations [Member] | Weighted Average
Quantitative information for Level 3 Fair Value Measurements Liabilities
Foreign exchange volatilities (as a percent)	2.10%
Equity Index Options
Quantitative information for Level 3 Fair Value Measurements Liabilities
Fair value Liabilities	$ 57.3
Equity Index Options | Counterparty valuations [Member] | Minimum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Expected equity dividends (as a percent)	1.70%
Equity Index Options | Counterparty valuations [Member] | Maximum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Expected equity dividends (as a percent)	3.00%
Equity Index Options | Counterparty valuations [Member] | Weighted Average
Quantitative information for Level 3 Fair Value Measurements Liabilities
Expected equity dividends (as a percent)	2.50%
Year One | Variable annuity benefit guarantee liabilities | Discounted cash flows | Minimum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Surrenders (as a percent)	0.30%
Foreign exchange volatilities (as a percent)	13.20%
Index volatilities (as a percent)	15.50%
Year One | Variable annuity benefit guarantee liabilities | Discounted cash flows | Maximum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Surrenders (as a percent)	10.80%
Foreign exchange volatilities (as a percent)	14.80%
Index volatilities (as a percent)	20.00%
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)	1.30%
Foreign exchange volatilities (as a percent)	13.90%
Index volatilities (as a percent)	16.70%
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)	12.80%
Index volatilities (as a percent)	18.00%
Year Two | Variable annuity benefit guarantee liabilities | Discounted cash flows | Maximum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Surrenders (as a percent)	8.70%
Foreign exchange volatilities (as a percent)	16.00%
Index volatilities (as a percent)	21.00%
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)	1.90%
Foreign exchange volatilities (as a percent)	14.60%
Index volatilities (as a percent)	19.20%
Year Three and Thereafter | 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)	14.10%
Index volatilities (as a percent)	20.10%
Year Three and Thereafter | Variable annuity benefit guarantee liabilities | Discounted cash flows | Maximum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Surrenders (as a percent)	6.60%
Foreign exchange volatilities (as a percent)	17.60%
Index volatilities (as a percent)	24.20%
Year Three and Thereafter | Variable annuity benefit guarantee liabilities | Discounted cash flows | Weighted Average
Quantitative information for Level 3 Fair Value Measurements Liabilities
Surrenders (as a percent)	1.60%
Foreign exchange volatilities (as a percent)	15.30%
Index volatilities (as a percent)	22.40%