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Summary of Significant Accounting Policies (Details 2) (USD $) In Millions, except Share data, unless otherwise specified | 3 Months Ended |  |  |  |  |  |  | 1 Months Ended |  |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Mar. 31, 2012 | Mar. 31, 2011 | Jan. 02, 2012 | Dec. 31, 2011 | Mar. 31, 2012 OneBeacon Ltd. | Dec. 31, 2011 OneBeacon Ltd. | Mar. 31, 2012 Sirius Group | Dec. 31, 2011 Sirius Group | May 31, 2007 Sirius Group Non-cumulative perpetual preference shares | May 24, 2007 Sirius Group Non-cumulative perpetual preference shares | Mar. 31, 2012 White Mountains consolidated limited partnerships | Dec. 31, 2011 White Mountains consolidated limited partnerships | Mar. 31, 2012 A.W.G. Dewar Inc | Dec. 31, 2011 A.W.G. Dewar Inc | Mar. 31, 2012 Hamer and Bri-Mar | Mar. 31, 2012 Passage 2 Health Limited | Dec. 31, 2011 Passage 2 Health Limited
Noncontrolling Interest |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Noncontrolling equity percentage in OneBeacon Ltd. |  |  |  |  | 24.80% | 24.50% |  |  |  |  |  |  |  |  |  |  |
Non-cumulative perpetual preference shares issued |  |  |  |  |  |  |  |  | 250,000,000 |  |  |  |  |  |  |  |
Non-cumulative perpetual preference shares per share liquidation preference (in dollars per share) |  |  |  |  |  |  |  |  |  | $ 1,000 |  |  |  |  |  |  |
Proceeds from non-cumulative perpetual preference shares |  |  |  |  |  |  |  |  | $ 245.7 |  |  |  |  |  |  |  |
Non-cumulative perpetual preference shares issuance cost and commission |  |  |  |  |  |  |  |  |  | 4.3 |  |  |  |  |  |  |
Noncontrolling equity interest | 58.8 |  |  | 57.1 |  |  | 0.5 | 0.6 |  |  | 52.3 | 54.2 | 2.2 | 2.3 | 3.9 | 0.5 | 0.6
Deferred acquisition costs | 198.8 |  | 5.6 | 187.0 |  |  |  |  |  |  |  |  |  |  |  |  |
Deferred policy acquisition costs, amortization expense | $ 2.4 | $ 1.8 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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