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Summary of Significant Accounting Policies (Noncontrolling Interest) (Details) (USD $) In Millions, except Share data, unless otherwise specified | 3 Months Ended | 12 Months Ended |  |  |  |  |  |  |  |  | 9 Months Ended | 12 Months Ended | 1 Months Ended |  | 1 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Dec. 31, 2012 | Sep. 30, 2012 | Jun. 30, 2012 | Mar. 31, 2012 | Dec. 31, 2011 | Sep. 30, 2011 | Jun. 30, 2011 | Mar. 31, 2011 | Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2012 White Mountains Consolidated Limited Partnership | Dec. 31, 2011 White Mountains Consolidated Limited Partnership | Dec. 31, 2012 A W G Dewar Inc | Dec. 31, 2011 A W G Dewar Inc | Dec. 31, 2012 Passage 2 Health Limited | Dec. 31, 2011 Passage 2 Health Limited | Dec. 31, 2012 OneBeacon Ltd. | Dec. 31, 2011 OneBeacon Ltd. | Sep. 30, 2012 White Mountains | Dec. 31, 2012 White Mountains | Dec. 31, 2012 HG Global | May 31, 2007 Preferred stocks Sirius Internation Group Ltd quarter | May 24, 2007 Preferred stocks Sirius Internation Group Ltd | Jul. 31, 2012 BAM Management HG Global | Dec. 31, 2012 Non-controlling Interests HG Global | Dec. 31, 2012 BAM Non-controlling Interests Build America Mutual
Noncontrolling Interest [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Pre-tax income (loss) | $ (50.8) | $ 135.1 | $ 37.6 | $ 140.9 | $ 98.4 | $ 1.2 | $ 35.8 | $ (37.4) | $ 262.8 | $ 98.0 | $ 189.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ (36.3)
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 |  |  |
Preferred Stock Redemption Basis Spread on Treasury Issue Rate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 45.00% |  |  |  |
Proceeds from non-cumulative perpetual preference shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 245.7 |  |  |  |
Non-cumulative perpetual preference shares issuance cost and commission |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 4.3 |  |  |
Right to elect eirectors on board number |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2 |  |  |  |
Period for non payment of dividend which give right to elect directors on board |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 6 |  |  |  |
Minimum required payment period |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 4 |  |  |  |
Preferred stock dividend rate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 6.00% | 7.506% |  |  |  |
Preference shares reference rate for dividend |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3 month LIBOR |  |  |  |
Floating annual dividend rate fixed |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 7.506% |  |  |  |
Preference shares floating rate of dividend amount in excess of libor |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3.20% |  |  |  |
Noncontrolling interest—consolidated limited partnerships |  |  |  |  |  |  |  |  |  |  |  | 41.5 | 54.2 | 2.8 | 2.3 | 0.2 | 0.6 |  |  |  |  |  |  |  |  |  |
Capital contributions |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 594.5 |  |  |  |  |  |  |
Funds to capitalize HG Global |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 600.0 | 594.5 |  |  |  | 14.5 |  |
Fair Value of Net Assets Acquired |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ (2.2) |

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