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Loss and Loss Adjustment Expense Reserves Details (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2012 | Sep. 30, 2011 | Sep. 30, 2012 | Sep. 30, 2011
Loss and Loss Adjustment, Expense Reserves [Line Items] |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims, Prior Years | $ 10.1 | $ 26.9 | $ 21.2 | $ 47.6
Accretion of fair value adjustment to net loss and LAE reserves | 1.1 | 2.1 | 9.4 | 6.2
Accretion of fair value adjustment to net loss and LAE reserves increase |  |  | 5.0 |
Remaining unamortized fair value adjustment |  |  | 2.0 |
One Beacon Segment [Member] |  |  |  |
Loss and Loss Adjustment, Expense Reserves [Line Items] |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims, Prior Years | (2.3) | (6.0) | (7.6) | (14.4)
Sirius |  |  |  |
Loss and Loss Adjustment, Expense Reserves [Line Items] |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims, Prior Years | (7.8) | (20.9) | (13.6) | (33.2)
Net Unfavorable Loss Reserve Development Due To Asbestos And Environmental Reserves Additions | $ 33.0 |  | $ 45.0 |

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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AICPA -Name Statement of Position (SOP) -Number 94-5 -Paragraph 11 -Subparagraph b -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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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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Data Type: | xbrli:monetaryItemType
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- Details
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- Definition Represents the net unfavorable loss reserve development due to additions to the Asbestos and Environmental reserves. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Represents the remaining unamortized amount of accreted fair value adjustment to loss and LAE reserves related to purchase accounting. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
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