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Loss and Loss Adjustment Expense Reserves Details (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 6 Months Ended | 3 Months Ended | 6 Months Ended | 3 Months Ended | 6 Months Ended | 3 Months Ended
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Jun. 30, 2013 | Jun. 30, 2012 | Jun. 30, 2013 | Jun. 30, 2012 | Jun. 30, 2013 One Beacon | Jun. 30, 2012 One Beacon | Jun. 30, 2013 One Beacon | Jun. 30, 2012 One Beacon | Jun. 30, 2013 Sirius Group [Member] | Jun. 30, 2013 Sirius Group [Member] | Jun. 30, 2012 Sirius Group [Member] | Mar. 31, 2012 Scandinavian Reinsurance Company [Member]
Loss and Loss Adjustment, Expense Reserves [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net favorable and unfavorable loss reserve development | $ (15.4) | $ (3.5) | $ (14.0) | $ (11.1) | $ (1.0) | $ (3.4) | $ (3.9) | $ (5.3) | $ (14.4) | $ (10.1) | $ (5.8) |
Accretion of fair value adjustment to net loss and LAE reserves | 0.1 | 1.2 | 1.3 | 8.3 |  |  |  |  |  |  |  |
Accretion of fair value adjustment to net loss and LAE reserves increase |  |  |  |  |  |  |  |  |  |  |  | 5.0
Adjustment to loss and loss adjustment expense reserves, before tax | $ 4.4 |  | $ 4.4 |  |  |  |  |  |  |  |  |

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- Definition Amount of provision in the period for claims incurred in prior reporting periods and related claims settlement costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Financial Reporting Release (FRR) -Number 403 -Paragraph 04 -Subparagraph b Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Statement of Position (SOP) -Number 94-5 -Paragraph 11 -Subparagraph b -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
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- Definition Represents the amount of accreted fair value adjustment to loss and LAE reserves related to purchase accounting. + References No definition available. + Details
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- Definition Represents the amount of accreted fair value adjustment to loss and LAE reserves related to purchase accounting. + References No definition available. + Details
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- Definition Represents the amount of pre-tax unaccreted fair value adjustment to loss and LAE reserves related to purchase accounting. + References No definition available. + Details
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