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Summary of Significant Accounting Policies (Basis of Presentation) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified |  |  |  |  | 3 Months Ended | 9 Months Ended | 1 Months Ended | 3 Months Ended | 9 Months Ended |  |  |  |  |  |  | 1 Months Ended |  |  |  |  |  | 1 Months Ended |  | 1 Months Ended | 3 Months Ended | 9 Months Ended |
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Sep. 30, 2014 | Dec. 31, 2013 | Sep. 30, 2014 OneBeacon Ltd. | Dec. 31, 2013 OneBeacon Ltd. | Sep. 30, 2014 Build America Mutual | Sep. 30, 2013 Build America Mutual | Sep. 30, 2014 Build America Mutual | Sep. 30, 2013 Build America Mutual | Jul. 31, 2012 HG Global | Sep. 30, 2014 HG Global | Sep. 30, 2013 HG Global | Sep. 30, 2014 HG Global | Sep. 30, 2013 HG Global | Dec. 31, 2013 HG Global | Sep. 30, 2014 Star & Shield Insurance Exchange | Sep. 30, 2014 White Mountains Consolidated Limited Partnership | Dec. 31, 2013 White Mountains Consolidated Limited Partnership | Sep. 30, 2014 A W G Dewar Inc | Dec. 31, 2013 A W G Dewar Inc | Feb. 19, 2014 WOBI | May 31, 2007 Preferred Stock Sirius International Group Ltd | May 24, 2007 Preferred Stock Sirius International Group Ltd | Sep. 30, 2014 Preferred Stock HG Global | Dec. 31, 2013 Preferred Stock HG Global | Sep. 30, 2014 Common Stock HG Global | Dec. 31, 2013 Common Stock HG Global | Jul. 31, 2012 Common shares and paid-in surplus HG Global | Sep. 30, 2014 Non-controlling interest Star & Shield Insurance Exchange | Sep. 30, 2014 Non-controlling interest WOBI | Sep. 30, 2014 Non-controlling interest QuoteLab | Jul. 31, 2012 BAM Management | Sep. 30, 2013 Interest Rate Cap Sirius International Group Ltd | Sep. 30, 2014 Interest Rate Cap Sirius International Group Ltd | Jul. 30, 2013 Interest Rate Cap Sirius International Group Ltd
Basis of Presentation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest (as a percent) |  |  | 75.30% | 75.20% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 60.70% |  |  | 96.90% |  | 88.40% |  |  |  |  |  |  |  |  |
Surplus Notes |  |  |  |  |  |  |  |  | $ 503.0 |  |  |  |  |  | $ 17.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Partners Capital Contribution |  |  |  |  |  |  |  |  | 594.5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 14.5 |  |  |
Percentage of par value of policy reinsured |  |  |  |  |  |  |  |  |  | 15.00% |  | 15.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Noncontrolling equity percentage |  |  | 24.70% | 24.80% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 96.90% | 97.30% | 88.40% | 88.70% |  |  |  |  |  |  |  |
Fair Value of Net Assets Acquired |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2.2 |  |  |  |  |  |  |
Income (Loss) from Continuing Operations before Income Taxes, Domestic |  |  |  |  | (13.1) | (15.1) | (29.7) | (60.2) |  | 3.8 | 10.5 | 13.4 | 28.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Non-cumulative perpetual preference shares issued |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 250.0 |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |  |  |  |
Preferred Stock, Dividend Rate, Percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 7.506% |  |  |  |  |  |  |  |  |  |  |  |  |
basis points |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3.20% |  |  |  |  |  |  |  |  |  |  |  | 3.95% |
years of interest rate cap |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5 years |  |
Derivative, Cap Interest Rate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 8.30%
Noncontrolling interest - other |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 45.5 | 46.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Non-controlling equity interest | $ 488.7 | $ 491.9 |  |  |  |  |  |  |  | $ 18.0 |  | $ 18.0 |  | $ 16.6 |  |  |  | $ 3.2 | $ 3.1 |  |  |  |  |  |  |  |  | $ (11.1) | $ 3.8 | $ 18.0 |  |  |  |
Term of LIBOR loan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3 months |  |  |  |  |  |  |  |  |  |  |  |  |

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