Total number of common shares of an entity that have been sold or granted to shareholders (includes common shares that were issued, repurchased and remain in the treasury). These shares represent capital invested by the firm's shareholders and owners, and may be all or only a portion of the number of shares authorized. Shares issued include shares outstanding and shares held in the treasury.

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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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Carrying value of a distribution

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

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Loss resulting from the dilutive effect of dividend payments and issuance of restricted shares by an equity method investee.

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Investments in Unconsolidated Affiliates (Investments in Symetra) (Details) - USD ($) $ / shares in Units, $ in Millions			3 Months Ended								10 Months Ended	12 Months Ended
	Nov. 05, 2015	Dec. 31, 2014	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Nov. 05, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Changes in carrying value of investment in Symetra
Equity in earnings of unconsolidated affiliates, net of tax			$ 0.0	$ 0.0	$ 0.0	$ 0.0	$ 7.1	$ 3.9	$ 6.8	$ 7.3		$ 0.0	$ 25.1	$ 45.6
Net change in pre-tax unrealized investment (losses) gains on investments in unconsolidated affiliates												0.0	(39.2)	81.2
Income Tax Expense (Benefit)			$ 23.0	$ 6.7	$ 6.0	$ 9.7	$ 1.0	$ 1.6	$ 2.2	$ (4.6)		$ 45.4	$ 0.2	14.8
Common shares, issued shares			4,563,814				5,623,735					4,563,814	5,623,735
Common Stock \| Symetra
Changes in carrying value of investment in Symetra
Equity in earnings of unconsolidated affiliates, net of tax											$ 25.1
Net change in pre-tax unrealized investment (losses) gains on investments in unconsolidated affiliates											(39.2)
Carrying value of a distribution											12.4
Dividends received											(16.9)
Carrying value at the end of the period	$ 392.8	$ 411.4									392.8			411.4
Loss from dilutive effect resulting from dividend and issuance of restricted shares											(0.1)
Aggregate value of White Mountains common shares of Symetra at quoted market price	$ 651.2										$ 651.2
Share Price	$ 31.67										$ 31.67
Common Stock \| White Mountains
Changes in carrying value of investment in Symetra
Net change in pre-tax unrealized investment (losses) gains on investments in unconsolidated affiliates	$ (1.6)	$ 37.6
equity in earnings of unconsolidated affiliates [Member] \| Common Stock \| Symetra
Changes in carrying value of investment in Symetra
Income Tax Expense (Benefit)											$ (1.6)
Net unrealized gains (losses) of unconsolidated affiliates [Member] \| Common Stock \| Symetra
Changes in carrying value of investment in Symetra
Income Tax Expense (Benefit)											2.9
Net unrealized gains (losses) of unconsolidated affiliates [Member] \| Common Stock \| White Mountains
Changes in carrying value of investment in Symetra
Income Tax Expense (Benefit)											0.2			$ (2.7)
impairment of net unrealized gains (losses) [Member] \| Net unrealized gains (losses) of unconsolidated affiliates [Member] \| Common Stock \| Symetra
Changes in carrying value of investment in Symetra
Net change in pre-tax unrealized investment (losses) gains on investments in unconsolidated affiliates											9.4
Amortization of common share basis difference [Member] \| Common Stock \| Symetra
Changes in carrying value of investment in Symetra
Equity in earnings of unconsolidated affiliates, net of tax											$ 2.3
Prospector
Changes in carrying value of investment in Symetra
Common shares, issued shares							513,500						513,500