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Summary of Significant Accounting Policies (Basis of Presentation) (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | ---
Sep. 30, 2015 | Jun. 30, 2015 | Sep. 30, 2014 | Sep. 30, 2015 | Sep. 30, 2014 | Dec. 31, 2014 | Dec. 31, 2013 | Jun. 30, 2014 | Jul. 31, 2012
Basis of Presentation |  |  |  |  |  |  |  |  |
Net investment income | $ 16.8 |  | $ 13.8 | $ 42.6 | $ 44.7 |  |  |  |
Income Tax Expense (Benefit) | $ (1.6) |  | (4.7) | $ 0.8 | 11.4 |  |  |  |
OneBeacon Ltd. |  |  |  |  |  |  |  |  |
Basis of Presentation |  |  |  |  |  |  |  |  |
Cumulative Effect on Retained Earnings, Net of Tax |  |  |  |  |  | $ 0.9 | $ 0.5 |  |
Ownership interest (as a percent) | 75.30% |  |  | 75.30% |  | 75.30% |  |  |
HG Global |  |  |  |  |  |  |  |  |
Basis of Presentation |  |  |  |  |  |  |  |  |
Surplus Notes |  |  |  |  |  |  |  |  | $ 503.0
Percentage of par value of policy reinsured | 15.00% |  |  | 15.00% |  |  |  |  |
Noncontrolling equity percentage | 3.10% |  |  | 3.10% |  | 3.10% |  |  |
Star & Shield Insurance Exchange |  |  |  |  |  |  |  |  |
Basis of Presentation |  |  |  |  |  |  |  |  |
Surplus Notes | $ 20.0 |  |  | $ 20.0 |  |  |  |  |
Noncontrolling equity percentage | 100.00% |  |  | 100.00% |  | 100.00% |  |  |
White Mountains Advisors L L C [Member] |  |  |  |  |  |  |  |  |
Basis of Presentation |  |  |  |  |  |  |  |  |
Investment Advisory, Management and Administrative Fees | $ 2.0 |  | 3.2 | $ 5.9 | 9.2 |  |  |  |
Preferred Stock | HG Global |  |  |  |  |  |  |  |  |
Basis of Presentation |  |  |  |  |  |  |  |  |
Ownership interest (as a percent) | 96.90% |  |  | 96.90% |  |  |  |  |
Noncontrolling equity percentage | 96.90% |  |  | 96.90% |  | 96.90% |  |  |
Common Stock | HG Global |  |  |  |  |  |  |  |  |
Basis of Presentation |  |  |  |  |  |  |  |  |
Ownership interest (as a percent) | 88.40% |  |  | 88.40% |  |  |  |  |
Noncontrolling equity percentage | 88.40% |  |  | 88.40% |  | 88.40% |  |  |
Accounting Standards Update 2014-01 [Member] | Restatement Adjustment [Member] | OneBeacon Ltd. |  |  |  |  |  |  |  |  |
Basis of Presentation |  |  |  |  |  |  |  |  |
Net investment income |  |  | 0.4 |  | 1.3 |  |  |  |
Income Tax Expense (Benefit) |  |  | $ 0.6 |  | $ 1.8 |  |  |  |
OneBeacon Runoff [Member] |  |  |  |  |  |  |  |  |
Basis of Presentation |  |  |  |  |  |  |  |  |
Proceeds from Sale of Buildings |  | $ 58.0 |  |  |  |  |  |  |
Surplus Notes |  |  |  |  |  |  |  | $ 80.9 |

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