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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
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-Publisher SEC
-Name Regulation S-X (SX)
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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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Price of a single share of a number of saleable stocks of a company.

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No definition available.

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Carrying value of a distribution

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Represents the equity in unrealized gains or losses from investments in unconsolidated affiliates.

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No definition available.

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- Definition

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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No definition available.

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

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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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No definition available.

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Investments in Unconsolidated Affiliates (Investments in Symetra) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								6 Months Ended	10 Months Ended	12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Jun. 30, 2013	Nov. 05, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Changes in carrying value of investment in Symetra
Equity in earnings of unconsolidated affiliates, net of tax	$ 7.1	$ 3.9	$ 6.8	$ 7.3	$ 12.3	$ 7.0	$ 12.5	$ 13.8			$ 25.1	$ 45.6	$ 36.6
Net change in pre-tax unrealized gains (losses) on investments in unconsolidated affiliates											(39.2)	81.2	(106.4)
Carrying value at the end of the period	0.0				414.4					$ 392.8	0.0	414.4
Tax benefit (expense)	$ 1.5	$ 1.6	$ 2.2	$ (4.6)	26.2	$ 4.7	$ (7.1)	$ (9.0)			$ 0.7	14.8	(32.6)
Symetra
Changes in carrying value of investment in Symetra
Equity in earnings of unconsolidated affiliates, net of tax										25.1		47.0	37.8
Net change in pre-tax unrealized gains (losses) on investments in unconsolidated affiliates										(39.2)		81.2	(106.4)
Dividends received										(16.9)		(34.1)	(6.4)
Increase (decrease) in value of warrants													10.8
Carrying value at the end of the period					411.4					392.8		411.4	317.3
Unrealized (losses) gains from Symetra’s fixed maturity portfolio					37.6					(1.6)		37.6
Exercise of warrants													0.0
Common Stock \| Symetra
Changes in carrying value of investment in Symetra
Equity in earnings of unconsolidated affiliates, net of tax										25.1		47.0	37.8
Net change in pre-tax unrealized gains (losses) on investments in unconsolidated affiliates										(39.2)		81.2	(106.4)
Carrying value of a distribution										12.4
Dividends received										(16.9)		(34.1)	(6.4)
Increase (decrease) in value of warrants													0.0
Carrying value at the end of the period					411.4					392.8		411.4	317.3
Loss from dilutive effect resulting from dividend and issuance of restricted shares										(0.1)		(0.1)	0.2
Aggregate value of White Mountains common shares of Symetra at quoted market price										$ 651.2
Share Price										$ 31.67
Exercise of warrants													41.1
Common Stock \| White Mountains
Changes in carrying value of investment in Symetra
Net change in pre-tax unrealized gains (losses) on investments in unconsolidated affiliates										$ (1.6)		37.6	(43.6)
Warrants
Changes in carrying value of investment in Symetra
Increase (decrease) in value of warrants									$ 10.8
Warrants \| Symetra
Changes in carrying value of investment in Symetra
Equity in earnings of unconsolidated affiliates, net of tax										0.0		0.0	0.0
Net change in pre-tax unrealized gains (losses) on investments in unconsolidated affiliates										0.0		0.0	0.0
Dividends received										0.0		0.0	0.0
Increase (decrease) in value of warrants													10.8
Carrying value at the end of the period					$ 0.0					0.0		0.0	$ 0.0
Share Price													$ 11.49
Exercise of warrants													$ (41.1)
equity in earnings of unconsolidated affiliates [Member] \| Common Stock \| Symetra
Changes in carrying value of investment in Symetra
Tax benefit (expense)										(1.6)		(3.3)	(2.8)
Net unrealized gains (losses) of unconsolidated affiliates [Member] \| Common Stock \| Symetra
Changes in carrying value of investment in Symetra
Tax benefit (expense)										2.9		(5.9)	8.3
Net unrealized gains (losses) of unconsolidated affiliates [Member] \| Common Stock \| White Mountains
Changes in carrying value of investment in Symetra
Tax benefit (expense)										0.2		(2.7)	3.2
impairment of equity in earnings [Member] \| equity in earnings of unconsolidated affiliates [Member] \| Common Stock \| Symetra
Changes in carrying value of investment in Symetra
Equity in earnings of unconsolidated affiliates, net of tax													3.0
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 gains (losses) on investments in unconsolidated affiliates										9.4		12.7	$ 11.8
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		$ 3.0