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Summary of Significant Accounting Policies (Basis of Presentation) (Details) (USD $) In Millions, unless otherwise specified | 1 Months Ended | 3 Months Ended | 12 Months Ended |  |  |  | 9 Months Ended | 12 Months Ended |  | 12 Months Ended | 1 Months Ended | 12 Months Ended
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Dec. 31, 2011 company | Dec. 31, 2012 state | Sep. 30, 2012 | Jun. 30, 2012 | Mar. 31, 2012 | Dec. 31, 2011 | Sep. 30, 2011 | Jun. 30, 2011 | Mar. 31, 2011 | Dec. 31, 2012 state | Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2012 OneBeacon Ltd. | Dec. 31, 2011 OneBeacon Ltd. | Dec. 31, 2012 HG Global | Sep. 30, 2012 White Mountains | Dec. 31, 2012 White Mountains | Dec. 31, 2012 Redeemable preferred stocks HG Global | Dec. 31, 2012 Common shareholders’ equity HG Global | Dec. 31, 2011 Tuckerman Fund I | Dec. 31, 2010 Tuckerman Fund I | Dec. 31, 2011 Tuckerman Fund I Common shareholders’ equity | Dec. 31, 2011 Tuckerman Fund I Common shareholders’ equity
Basis of Presentation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership percentage in outstanding common shares |  |  |  |  |  |  |  |  |  |  |  |  | 75.20% | 75.50% |  |  |  |  |  |  |  |  |
Funds to capitalize HG Global |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 600.0 | $ 594.5 |  |  |  |  |  |
Surplus notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 503.0 |  |  |  |  |  |  |  |
Number of manufacturing companies | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 97.30% |  |  | 97.30% | 88.70% |  |  |  |
Ownership interest in VIE (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 94.00% | 94.00%
Percentage of par value of policy reinsured |  | 15.00% |  |  |  |  |  |  |  | 15.00% |  |  |  |  | 15.00% |  |  |  |  |  |  |  |
Number of states |  | 30 |  |  |  |  |  |  |  | 30 |  |  |  |  |  |  |  |  |  |  |  |  |
Pre-tax income (loss) |  | $ (12.0) | $ (30.9) | $ 12.1 | $ 16.8 | $ 20.3 | $ (11.0) | $ 20.8 | $ 11.4 | $ 14.0 | $ (41.5) | $ (53.0) |  |  |  |  |  |  |  | $ (2.2) | $ (1.0) |  |

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