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Investments in Unconsolidated Affiliates (Investments in Symetra) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified | 3 Months Ended
--- | ---
Mar. 31, 2014 | Mar. 31, 2013
Investments in unconsolidated affiliates |  |
Income Tax Expense (Benefit) | $ 30.3 | $ 41.6
Changes in carrying value of investment in Symetra |  |
Carrying value at the beginning of the period | 321.4 |
Equity in earnings | 13.8 | 9.2
Carrying value at the end of the period | 373.3 |
Symetra |  |
Changes in carrying value of investment in Symetra |  |
Carrying value at the beginning of the period | 317.3 | 381.5
Equity in earnings | 14.4 | 9.8
Equity in net unrealized losses from Symetra's fixed maturity portfolio | 39.3 | (8.7)
Dividends received | (2.0) | (1.4)
Increase (decrease) in value of warrants | 0 | (3.7)
Carrying value at the end of the period | 369.0 | 377.5
Symetra | Common shareholders' equity. |  |
Changes in carrying value of investment in Symetra |  |
Carrying value at the beginning of the period | 317.3 | 351.2
Equity in earnings | 14.4 | 9.8
Equity in net unrealized losses from Symetra's fixed maturity portfolio | 39.3 | (8.7)
Dividends received | (2.0) | (1.4)
Increase (decrease) in value of warrants | 0 | 0
Carrying value at the end of the period | 369.0 | 350.9
Aggregate value of White Mountains common shares of Symetra at quoted market price | 397.4 |
Share Price | $ 19.82 |
Symetra | Warrants |  |
Changes in carrying value of investment in Symetra |  |
Carrying value at the beginning of the period | 0 | 30.3
Equity in earnings | 0 | 0
Equity in net unrealized losses from Symetra's fixed maturity portfolio | 0 | 0
Dividends received | 0 | 0
Increase (decrease) in value of warrants | 0 | (3.7)
Carrying value at the end of the period | 0 | 26.6
Share Price |  | $ 13.41
White Mountains | Common shareholders' equity. |  |
Changes in carrying value of investment in Symetra |  |
Equity in net unrealized losses from Symetra's fixed maturity portfolio | 39.3 | (8.7)
equity in earnings of unconsolidated affiliates [Member] | Symetra | Common shareholders' equity. |  |
Investments in unconsolidated affiliates |  |
Income Tax Expense (Benefit) | 1.0 | 0.8
Net unrealized gains (losses) of unconsolidated affiliates [Member] | White Mountains | Common shareholders' equity. |  |
Investments in unconsolidated affiliates |  |
Income Tax Expense (Benefit) | (2.8) | 0.7
Amortization of common share basis difference [Member] | Symetra | Common shareholders' equity. |  |
Changes in carrying value of investment in Symetra |  |
Equity in earnings | 0.7 | 0.7
impairment of net unrealized gains (losses) [Member] | Net unrealized gains (losses) of unconsolidated affiliates [Member] | 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 | $ 2.7

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- Definition 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=31042434&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 9 -Article 5 Reference 3: 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=16385135&loc=d3e33749-111570 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.12) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 11 -Article 7 + Details
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- Definition Price of a single share of a number of saleable stocks of a company. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258 + Details
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- Definition 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. + References No definition available. + Details
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