Derivatives (Level 3 Derivative Instruments) (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2013
Variable annuity benefit guarantee liabilities
Quantitative information for Level 3 Fair Value Measurements Liabilities
Fair value Liabilities	$ 143.7
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)	5.30%
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 Options
Quantitative information for Level 3 Fair Value Measurements Liabilities
Fair value Liabilities	57.8
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	$ 44.6
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)	2.60%
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)	11.20%
Index volatilities (as a percent)	15.60%
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)	13.50%
Index volatilities (as a percent)	19.60%
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)	2.40%
Foreign exchange volatilities (as a percent)	12.00%
Index volatilities (as a percent)	16.60%
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)	12.30%
Index volatilities (as a percent)	17.60%
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)	14.50%
Index volatilities (as a percent)	21.40%
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)	2.60%
Foreign exchange volatilities (as a percent)	13.20%
Index volatilities (as a percent)	19.00%
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)	13.80%
Index volatilities (as a percent)	19.60%
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)	22.40%
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)	1.90%
Foreign exchange volatilities (as a percent)	14.70%
Index volatilities (as a percent)	21.30%