Derivatives (Pre-Tax Operating Results of WM Life Re) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Pre-tax operating results of WM Life Re
Total revenues	$ 656.8us-gaap_Revenues	$ 578.7us-gaap_Revenues	$ 578.7us-gaap_Revenues
General and administrative expenses	(116.4)us-gaap_GeneralAndAdministrativeExpense	(50.3)us-gaap_GeneralAndAdministrativeExpense	(50.3)us-gaap_GeneralAndAdministrativeExpense
Pre-tax income from continuing operations	86.8	112.5	112.5
White Mountains Life Re
Pre-tax operating results of WM Life Re
Fees, included in other revenue	3.0	5.4
Change in fair value of variable annuity liability, included in other revenue	0.4	(1.3)
Realized and unrealized gains (losses)	(5.5)	(6.6)
Foreign exchange, included in other revenue	(1.1)	0.2
Other investment income and (losses) gains	0	0.4
Total revenues	(3.2)	(1.9)
Change in fair value of variable annuity death benefit liabilities, included in other general and administrative expenses	0	0
Death benefit claims paid, included in general and administrative expenses	0	0
General and administrative expenses	(1.4)	(1.3)
Pre-tax income from continuing operations	$ (4.6)	$ (3.2)

Amount of increase (decrease) in the fair value of derivatives recognized in the income statement.

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The aggregate total of expenses of managing and administering the affairs of an entity, including affiliates of the reporting entity, which are not directly or indirectly associated with the manufacture, sale or creation of a product or product line.

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Income from agency and brokerage operations (includes sales of annuities and supplemental contracts); service charges, commissions, and fees from the sale of insurance and related services; and management fees from separate accounts, deferred annuities, and universal life products.

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Monetary decrease in the future benefit reserve resulting from amounts paid to policy and contract holders during the period for policy benefit claims.

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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This item represents the change in fair value of derivatives due to changes in foreign currency exchange rates, which is included in other revenues.

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Net increase or decrease in the fair value of the variable annuity death benefit liabilities included in earnings.

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