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Income Taxes (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 9 Months Ended |  | 9 Months Ended
--- | --- | --- | --- | ---
Sep. 30, 2014 | Sep. 30, 2013 | Sep. 30, 2014 | Sep. 30, 2013 | Jul. 28, 2011 Tax Assessment Year 2007 to 2009 Internal Revenue Service (IRS) | Sep. 30, 2014 OneBeacon
Income Tax Contingency [Line Items] |  |  |  |  |  |
Effective tax rates (as a percent) | 22.70% | 14.80% | 24.30% | 23.40% |  |
U.S. statutory rate (as a percent) | 35.00% | 35.00% | 35.00% | 35.00% |  |
Income Tax Expense (Benefit) | $ 7.8 | $ 8.2 | $ 62.0 | $ 49.2 |  | $ (5.0)
Total tax assessment |  |  |  |  | 72.0 |
Tax assessments related to disallowed deductions |  |  |  |  | $ 60.2 |

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