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Investments in Unconsolidated Affiliates (Details 3) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Changes in carrying value of investment in Symetra
Carrying value at the beginning of the period	$ 275.3
Equity in earnings	10.2	6.7
Carrying value at the end of the period	294.9
Symetra
Changes in carrying value of investment in Symetra
Carrying value at the beginning of the period	273.6	387.5
Equity in earnings	10.9	7.3
Equity in net unrealized losses from Symetra's fixed maturity portfolio	(1.2)
Dividends received	(1.2)	(0.8)
Increase (decrease) in value of warrants	11.1	(1.2)
Carrying value at the end of the period	293.2	392.8
Tax on equity in earnings	0.7	0.6
Tax on equity in net unrealized gains (losses)	0	5.2
Aggregate value of White Mountains common shares of Symetra at quoted market price	200.6
Quoted market price (in dollars per share)	$ 11.53
Cash dividends received on warrants	1.2
Symetra | Common shareholders' equity.
Changes in carrying value of investment in Symetra
Carrying value at the beginning of the period	261.0	350.4
Equity in earnings	10.9	7.3
Equity in net unrealized losses from Symetra's fixed maturity portfolio	(1.2)
Dividends received	(1.2)	(0.8)
Carrying value at the end of the period	269.5	356.9
Equity in earnings increase relating to amortization of common share impairment	1.1
Equity in net unrealized losses increase relating to amortization of common share impairment	3.2
Symetra | Warrants
Changes in carrying value of investment in Symetra
Carrying value at the beginning of the period	12.6	37.1
Increase (decrease) in value of warrants	11.1	(1.2)
Carrying value at the end of the period	23.7	35.9
Cash dividends received on warrants	$ 0.7

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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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Dividends received on equity and other investments during the current period.

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Tax effect related to the accumulated equity in unrealized gains or losses from investments in unconsolidated affiliates.

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

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

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Represents the increase to equity in earnings from the amortization of the after-tax loss from the impairment of Symetra common shares.

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Represents the increase to other comprehensive income from the amortization of the after-tax loss from the impairment of Symetra common shares.

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Tax effect on the equity in earnings of an unconsolidated affiliate.

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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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Represents the market price per share.

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