Investments in Unconsolidated Affiliates (Investments in Symetra Table Rollforward) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Changes in carrying value of investment in Symetra
Carrying value at the beginning of the period	$ 414.4
Equity in earnings	7.3	13.8
Carrying value at the end of the period	450.8
Income Tax Expense (Benefit)	25.0us-gaap_IncomeTaxExpenseBenefit	30.9us-gaap_IncomeTaxExpenseBenefit
Symetra \| Common Stock
Investments in unconsolidated affiliates
Share Price	$ 23.46
Changes in carrying value of investment in Symetra
Carrying value at the beginning of the period	411.4	317.3
Equity in earnings	7.2	14.4
Equity in net unrealized gains (losses) from Symetra's fixed maturity portfolio	30.9	39.3
Dividends received	(2.2)	(2.0)
Carrying value at the end of the period	447.3	369.0
Aggregate value of White Mountains common shares of Symetra at quoted market price	470.3
White Mountains \| Common Stock
Changes in carrying value of investment in Symetra
Equity in net unrealized gains (losses) from Symetra's fixed maturity portfolio	68.4	39.3
equity in earnings of unconsolidated affiliates \| Symetra \| Common Stock
Changes in carrying value of investment in Symetra
Income Tax Expense (Benefit)	0.4	1.0
Net unrealized gains (losses) of unconsolidated affiliates \| White Mountains \| Common Stock
Changes in carrying value of investment in Symetra
Income Tax Expense (Benefit)	4.6	2.8
Amortization of common share basis difference \| Symetra \| Common Stock
Changes in carrying value of investment in Symetra
Equity in earnings	0.7	0.7
impairment of net unrealized gains (losses) \| Net unrealized gains (losses) of unconsolidated affiliates \| Symetra \| Common Stock
Changes in carrying value of investment in Symetra
Equity in net unrealized gains (losses) from Symetra's fixed maturity portfolio	$ 2.9	$ 3.1

This item represents disclosure of the amount of dividends or other distributions received from unconsolidated subsidiaries, certain corporate joint ventures, and certain noncontrolled corporation; these investments are accounted for under the equity method of accounting. This element excludes distributions that constitute a return of investment, which are classified as investing activities.

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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Price of a single share of a number of saleable stocks of a company.

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The net unrealized gains (losses) from Symetra's fixed maturity portfolio.

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Total investments in (A) companies that are accounted for using the equity method and (B) warrants to acquire additional shares of an unconsolidated affiliate that are carried at fair value.

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