Investments in Unconsolidated Affiliates (Investments in Symetra Table Rollforward) (Details) - USD ($)	3 Months Ended
Mar. 31, 2016	Mar. 31, 2015
Changes in carrying value of investment in Symetra
Equity in earnings	$ 0	$ 7,300,000
Income Tax Expense (Benefit)	(8,700,000)	4,600,000
Symetra \| Common Stock
Changes in carrying value of investment in Symetra
Carrying value at the beginning of the period		411,400,000
Equity in earnings		7,200,000
Equity in net unrealized gains (losses) from Symetra's fixed maturity portfolio		30,900,000
Dividends received		(2,200,000)
Carrying value at the end of the period		447,300,000
White Mountains \| Common Stock
Changes in carrying value of investment in Symetra
Equity in net unrealized gains (losses) from Symetra's fixed maturity portfolio		68,400,000
equity in earnings of unconsolidated affiliates \| Symetra \| Common Stock
Changes in carrying value of investment in Symetra
Income Tax Expense (Benefit)		400,000
Net unrealized gains (losses) of unconsolidated affiliates \| White Mountains \| Common Stock
Changes in carrying value of investment in Symetra
Income Tax Expense (Benefit)		(4,600,000)
Amortization of common share basis difference \| Symetra \| Common Stock
Changes in carrying value of investment in Symetra
Equity in earnings	$ 2,015	700,000
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		$ 2,900,000

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3602-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=35755396&loc=d3e33749-111570

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
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-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 03
-Paragraph 9
-Article 5

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 04
-Paragraph 11
-Article 7

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (a),(b)
-URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32639-109319

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(h))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Income Tax Expense (or Benefit)
-URI http://asc.fasb.org/extlink&oid=6515339

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

+ References

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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