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Summary of Significant Accounting Policies (Basis of Presentation) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified | 1 Months Ended | 3 Months Ended | 6 Months Ended |  |  | 1 Months Ended | 3 Months Ended | 12 Months Ended |  |  |  |  |  |  |  | 1 Months Ended |  |  |  |  |  | 12 Months Ended | 3 Months Ended | 6 Months Ended | 1 Months Ended
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Dec. 31, 2011 company | Jun. 30, 2013 | Jun. 30, 2012 | Jun. 30, 2013 | Jun. 30, 2012 | Jun. 30, 2013 OneBeacon Ltd. | Dec. 31, 2012 OneBeacon Ltd. | Jul. 31, 2012 White Mountains | Sep. 30, 2012 White Mountains | Dec. 31, 2012 White Mountains | Dec. 31, 2012 HG Global | Jun. 30, 2013 White Mountains Consolidated Limited Partnership | Dec. 31, 2012 White Mountains Consolidated Limited Partnership | Jun. 30, 2013 A W G Dewar Inc | Dec. 31, 2012 A W G Dewar Inc | Jun. 30, 2013 Passage 2 Health Limited | Dec. 31, 2012 Passage 2 Health Limited | May 31, 2007 Preferred Stock Sirius International Group Ltd | May 24, 2007 Preferred Stock Sirius International Group Ltd | Jun. 30, 2013 Preferred Stock HG Global | Dec. 31, 2012 Preferred Stock HG Global | Jun. 30, 2013 Common Stock HG Global | Dec. 31, 2012 Common Stock HG Global | Dec. 31, 2012 Common shares and paid-in surplus HG Global | Jun. 30, 2013 Noncontrolling interest Build America Mutual | Jun. 30, 2013 Noncontrolling interest Build America Mutual | Dec. 31, 2011 Tuckerman Fund I Common Stock | Jul. 31, 2012 BAM Management HG Global
Basis of Presentation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership percentage in outstanding common shares |  |  |  |  |  | 75.20% | 75.20% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Funds to Capitalize HG Global |  |  |  |  |  |  |  |  | $ 600.0 | $ 594.5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 14.5
Surplus Notes |  |  |  |  |  |  |  |  |  |  | 503.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of par value of policy reinsured |  | 15.00% |  | 15.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 15.00% |  |  |  |  |  |
Ownership interest in VIE (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 94.00% |
Number of small manufacturing companies owned by the Tuckerman Fund I from whom the entity received shares | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Noncontrolling equity percentage in OneBeacon Ltd. |  |  |  |  |  | 24.80% | 24.80% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Funds to Capitalized HG Global |  |  |  |  |  |  |  | 594.5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value of Net Assets Acquired |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2.2 |  |  |  |
Net loss (income) attributable to non-controlling interests |  | 11.1 | (12.1) | 11.6 | (28.9) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (20.9) | (41.0) |  |
Non-cumulative perpetual preference shares issued |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 250 |  |  |  |  |  |  |  |  |  |
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 interest - other |  |  |  |  |  |  |  |  |  |  |  | $ 45.0 | $ 41.5 | $ 2.8 | $ 2.8 | $ 0.1 | $ 0.2 |  |  |  |  |  |  |  |  |  |  |
Ownership interest (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 97.30% | 97.30% | 88.70% | 88.70% |  |  |  |  |

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