Investments in Unconsolidated Affiliates (Investments in Symetra) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Jun. 20, 2014
Changes in carrying value of investment in Symetra
Carrying value at the beginning of the period			$ 321.4
Equity in earnings	7.0	8.6	33.3	24.9
Carrying value at the end of the period	413.5		413.5
Income Tax Expense (Benefit)	7.8	8.2	62.0	49.2
Symetra
Changes in carrying value of investment in Symetra
Carrying value at the beginning of the period	415.5	326.0	317.3	381.5
Equity in earnings	7.0	8.7	34.5	25.7
Equity in net unrealized losses from Symetra's fixed maturity portfolio	(10.7)	(7.7)	64.0	(88.2)
Dividends received	(2.0)	(1.8)	(6.0)	(4.6)
Increase (decrease) in value of warrants	0	0	0	10.8
Carrying value at the end of the period	409.8	325.2	409.8	325.2
Exercise of warrants	0	0	0	0
Symetra \| Common shareholders' equity.
Investments in unconsolidated affiliates
Share Price	$ 22.70		$ 22.70
Changes in carrying value of investment in Symetra
Carrying value at the beginning of the period	415.5	326.0	317.3	351.2
Equity in earnings	7.0	8.7	34.5	25.7
Equity in net unrealized losses from Symetra's fixed maturity portfolio	(10.7)	(7.7)	64.0	(88.2)
Dividends received	(2.0)	(1.8)	(6.0)	(4.6)
Increase (decrease) in value of warrants	0	0	0	0
Carrying value at the end of the period	409.8	325.2	409.8	325.2
Aggregate value of White Mountains common shares of Symetra at quoted market price	455.1		455.1
Exercise of warrants	0	0	0	41.1
Symetra \| Warrants
Investments in unconsolidated affiliates
Share Price					$ 15.53
Changes in carrying value of investment in Symetra
Carrying value at the beginning of the period	0	0	0	30.3
Equity in earnings	0	0	0	0
Equity in net unrealized losses from Symetra's fixed maturity portfolio	0	0	0	0
Dividends received	0	0	0	0
Increase (decrease) in value of warrants	0	0	0	10.8
Carrying value at the end of the period	0	0	0	0
Exercise of warrants	0	0	0	(41.1)
White Mountains \| Common shareholders' equity.
Changes in carrying value of investment in Symetra
Equity in net unrealized losses from Symetra's fixed maturity portfolio	20.5	(25.5)	20.5	(25.5)
equity in earnings of unconsolidated affiliates \| Symetra \| Common shareholders' equity.
Changes in carrying value of investment in Symetra
Income Tax Expense (Benefit)	0.5	0.6	2.4	2.0
Net unrealized gains (losses) of unconsolidated affiliates \| White Mountains \| Common shareholders' equity.
Changes in carrying value of investment in Symetra
Income Tax Expense (Benefit)	(1.4)	2.0	(1.4)	2.0
Amortization of common share basis difference \| Symetra \| Common shareholders' equity.
Changes in carrying value of investment in Symetra
Equity in earnings	0.7	0.7	2.2	2.2
impairment of net unrealized gains (losses) \| Net unrealized gains (losses) of unconsolidated affiliates \| Symetra \| Common shareholders' equity.
Changes in carrying value of investment in Symetra
Equity in net unrealized losses from Symetra's fixed maturity portfolio	$ 3.1	$ 3.1	$ 9.4	$ 8.6

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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This item represents the entity's proportionate share for the period of the net income (loss) of its investee (such as unconsolidated subsidiaries and joint ventures) to which the equity method of accounting is applied. This item includes income or expense related to stock-based compensation based on the investor's grant of stock to employees of an equity method investee.

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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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No definition available.

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The value of the exercise of warrants.

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The increase (decrease) in the fair value of warrants included in other revenue.

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No definition available.

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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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Represents the aggregate value of common shares at quoted market price.

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