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Investments in Unconsolidated Affiliates (Details 2) (Symetra, USD $) In Millions, except Share data, unless otherwise specified | 3 Months Ended |  |  | 6 Months Ended | 1 Months Ended
--- | --- | --- | --- | --- | ---
Mar. 31, 2010 | Jun. 30, 2011 | Jun. 30, 2011 Common shareholders' equity | Dec. 31, 2010 Common shareholders' equity | Jun. 30, 2011 Warrants | Jan. 31, 2010 Initial public offering
Investments in unconsolidated affiliates |  |  |  |  |  |
Investments in unconsolidated affiliates (in shares) |  |  | 17,400,000 | 17,400,000 |  |
Additional common shares (in shares) |  |  | 9,500,000 | 9,500,000 |  |
Risk free interest rate (as a percent) |  |  |  |  | 0.84% |
Volatility rate (as a percent) |  |  |  |  | 25.00% |
Expected life (in years) |  |  |  |  | 3.08 |
Strike price (in dollars per share) |  |  |  |  | $ 11.49 |
Share price (in dollars per share) |  |  |  |  | $ 13.43 |
Initial public offering issue price (in dollars per share) |  |  |  |  |  | $ 12.00
Sale of Symetra common shares by Symetra (in shares) |  |  |  |  |  | 25,300,000
Sale of Symetra common shares by existing shareholders (in shares) |  |  |  |  |  | 9,700,000
Ownership interest after initial public offering (as a percent) | 20.00% |  |  |  |  |
Decrease in carrying value of Symetra from dilution | $ 16 |  |  |  |  |
Aggregate value of White Mountains common shares of Symetra at quoted market price |  | $ 233.7 |  |  |  |
Quoted market price (in dollars per share) |  | $ 13.43 |  |  |  |

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- Definition Percentage of subsidiary's or equity investee's stock owned by parent company after stock transaction. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 5 -Section H + Details
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- Definition The dollar amount received by subsidiary or equity investee for each share of common stock issued or sold in the stock transaction. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 5 -Section H + Details
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