Derivatives (Level 3 Derivative Instruments) (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013
Equity Index Options
Quantitative information for Level 3 Fair Value Measurements Liabilities
Fair value Liabilities	$ 30.8
Equity Index Options | Black-Scholes option pricing model | Minimum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Adjustment to counterparty valuations	0.50%
Equity Index Options | Black-Scholes option pricing model | Maximum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Adjustment to counterparty valuations	9.70%
Equity Index Options | Black-Scholes option pricing model | Weighted Average
Quantitative information for Level 3 Fair Value Measurements Liabilities
Adjustment to counterparty valuations	3.40%
Variable annuity benefit guarantee liabilities
Quantitative information for Level 3 Fair Value Measurements Liabilities
Fair value Liabilities	52.8
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	5.30%
Variable annuity benefit guarantee liabilities | Discounted cash flows | Weighted Average
Quantitative information for Level 3 Fair Value Measurements Liabilities
Mortality	1.00%
Foreign Exchange Options
Quantitative information for Level 3 Fair Value Measurements Liabilities
Fair value Liabilities	$ 32.6
Foreign Exchange Options | Black-Scholes option pricing model | Minimum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Foreign exchange volatilities	(1.20%)
Foreign Exchange Options | Black-Scholes option pricing model | Maximum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Foreign exchange volatilities	8.00%
Foreign Exchange Options | Black-Scholes option pricing model | Weighted Average
Quantitative information for Level 3 Fair Value Measurements Liabilities
Foreign exchange volatilities	2.40%
Year Three and Thereafter [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	13.50%
Index volatilities	18.90%
Year Three and Thereafter [Member] | Variable annuity benefit guarantee liabilities | Discounted cash flows | Maximum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Surrenders	14.00%
Foreign exchange volatilities	16.30%
Index volatilities	21.90%
Year Three and Thereafter [Member] | Variable annuity benefit guarantee liabilities | Discounted cash flows | Weighted Average
Quantitative information for Level 3 Fair Value Measurements Liabilities
Surrenders	6.20%
Foreign exchange volatilities	14.30%
Index volatilities	21.90%
Year Two [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	12.90%
Index volatilities	17.60%
Year Two [Member] | Variable annuity benefit guarantee liabilities | Discounted cash flows | Maximum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Surrenders	32.00%
Foreign exchange volatilities	15.00%
Index volatilities	20.10%
Year Two [Member] | Variable annuity benefit guarantee liabilities | Discounted cash flows | Weighted Average
Quantitative information for Level 3 Fair Value Measurements Liabilities
Surrenders	15.80%
Foreign exchange volatilities	13.70%
Index volatilities	19.40%
Year One [Member] | Variable annuity benefit guarantee liabilities | Discounted cash flows | Minimum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Surrenders	0.30%
Foreign exchange volatilities	11.60%
Index volatilities	16.40%
Year One [Member] | Variable annuity benefit guarantee liabilities | Discounted cash flows | Maximum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Surrenders	30.00%
Foreign exchange volatilities	14.00%
Index volatilities	19.50%
Year One [Member] | Variable annuity benefit guarantee liabilities | Discounted cash flows | Weighted Average
Quantitative information for Level 3 Fair Value Measurements Liabilities
Surrenders	21.00%
Foreign exchange volatilities	12.50%
Index volatilities	17.90%