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Reserves for Unpaid Losses and Loss Adjustment Expenses (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended |  | 12 Months Ended | 3 Months Ended | 6 Months Ended | 12 Months Ended |  | 12 Months Ended |  | 12 Months Ended |  | 12 Months Ended
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Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2009 | Dec. 31, 2012 Sirius Group | Dec. 31, 2011 Sirius Group | Dec. 31, 2010 Sirius Group | Dec. 31, 2012 Sirius Group | Sep. 30, 2012 Sirius Group | Mar. 31, 2012 Sirius Group | Jun. 30, 2012 Sirius Group | Dec. 31, 2012 Sirius Group Exposure | Dec. 31, 2011 Sirius Group | Dec. 31, 2010 Sirius Group | Dec. 31, 2009 Sirius Group | Dec. 31, 2012 OneBeacon | Dec. 31, 2011 OneBeacon | Dec. 31, 2010 OneBeacon | Dec. 31, 2012 Casualty Runoff Sirius Group | Dec. 31, 2012 Property Sirius Group | Dec. 31, 2012 Marine Energy Sirius Group | Dec. 31, 2012 Trade Credit Sirius Group | Dec. 31, 2012 Aviation Space Sirius Group | Dec. 31, 2012 Accident and Health [Member] Sirius Group | Dec. 31, 2011 Professional Liability [Member] OneBeacon | Dec. 31, 2009 Professional Liability [Member] OneBeacon | Dec. 31, 2011 Other Lines of Business OneBeacon | Dec. 31, 2010 Other Lines of Business OneBeacon | Dec. 31, 2011 Commercial Lines OneBeacon | Dec. 31, 2010 Commercial Lines OneBeacon | Dec. 31, 2009 Commercial Lines OneBeacon | Dec. 31, 2012 Asbestos Issue Sirius Group | Dec. 31, 2011 Asbestos Issue Sirius Group | Dec. 31, 2010 Asbestos Issue Sirius Group | Dec. 31, 2009 Asbestos Issue Sirius Group | Dec. 31, 2012 Environmental Issue Sirius Group | Dec. 31, 2011 Environmental Issue Sirius Group | Dec. 31, 2010 Environmental Issue Sirius Group | Dec. 31, 2009 Environmental Issue Sirius Group | Dec. 31, 2012 WM Solutions Sirius Group | Dec. 31, 2012 WM Solutions Sirius Group
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss and LAE incurred relating prior year losses | $ 41.9 | $ 76.7 | $ 93.1 |  | $ 34.5 | $ 46.9 | $ 57.1 |  |  |  |  |  |  |  |  | $ 7.4 | $ 29.8 | $ 36.0 | $ 31.5 | $ 28.4 | $ 11.6 | $ 6.8 | $ 5.3 | $ 4.3 | $ 11.5 | $ 19.3 | $ 4.3 | $ 9.2 |  | $ 7.5 | $ 7.5 |  |  |  |  |  |  |  |  |  |
Liability for asbestos and environmental claims, net, period increase (decrease) |  |  |  |  | 46.4 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Liability for environmental claims, net, incurred loss |  |  |  |  |  |  |  |  |  |  |  |  | 2.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (decrease) in reserves | 1,193.9 | 1,174.3 | 1,216.6 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 14.0 |  |  |  |  |  |  |  |  |  |  |  |
Net favorable loss reserve development due to property lines |  |  |  |  |  | 41.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net favorable loss reserve development due to 2010 Chilean earthquake |  |  |  |  |  | 13.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unfavorable loss reserve development due to asbestos and environmental reserves additions |  |  |  |  |  | 12.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss and loss adjustment expenses | 1,193.9 | 1,174.3 | 1,216.6 |  | 543.9 | 626.0 | 531.0 |  |  |  |  | 543.9 | 626.0 | 531.0 |  | 650.0 | 548.3 | 685.6 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Deferred gains recognized |  |  |  |  |  |  | 16.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accretion of fair value adjustment to net loss and LAE reserves | (10.6) | (8.3) | (8.5) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Un-accreted fair value, adjustment to loss and loss adjustment expense reserves, before tax | 5.7 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reinsurance recoverable on unpaid losses | 429.1 | 2,507.3 | 2,344.0 | 2,771.5 |  |  |  | 321.8 |  |  |  | 321.8 |  |  |  | 107.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Liability for asbestos and environmental claims |  |  |  |  |  |  |  | 189.4 |  |  |  | 189.4 | 162.7 | 169.6 | 164.5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 174.2 | 146.2 | 151.5 | 146.6 | 15.2 | 16.5 | 18.1 | 17.9 |  |
Asbestos and Environmental Loss Three Year Survival Ratio Period |  |  |  |  | 9 years | 11 years 1 month 6 days |  |  |  |  |  | 9 years | 11 years 1 month 6 days |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Liability for asbestos and environmental claims, gross |  |  |  |  |  |  |  | 228.6 |  |  |  | 228.6 | 207.2 | 214.3 | 209.2 | 1.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 208.2 | 185.1 | 191.9 | 187.0 | 20.4 | 22.1 | 22.4 | 22.2 |  |
Liabilty for asbestos claims, net, period increase (decrease) |  |  |  |  |  |  |  | 2.0 | 33.0 |  | 12.0 | 46.4 | 10.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 11.0 | 11.0
Liabilty for environmental claims, net, period increase (decrease) |  |  |  |  |  |  |  |  |  |  |  | 0.5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0.7 |
Liability for unpaid claims and claims adjustment expense |  |  |  |  |  |  |  |  |  | $ 8.5 |  | $ 30.9 | $ 19.2 | $ 13.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 29.4 | $ 15.6 | $ 12.1 |  | $ 1.5 | $ 3.6 | $ 0.9 |  |  |
Number of asbestos exposures |  |  |  |  | 5 |  |  |  |  |  |  | 5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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- Definition Carrying amount as of the balance sheet date of reserves for the costs of settling insured claims and costs incurred in the claims settlement process attributable to asbestos and environmental claims, before estimated recoveries from reinsurers. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Audit and Accounting Guide (AAG) -Number AAG-PLI -Paragraph 132, 133, 135, 136, 137 -IssueDate 2006-09-01 -Chapter 4 -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. Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Statement of Position (SOP) -Number 94-5 -Paragraph 11, 12 -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 Carrying amount as of the balance sheet date of reserves for the costs of settling insured claims and costs incurred in the claims settlement process attributable to asbestos and environmental claims, net of estimated reinsurance recoveries. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Audit and Accounting Guide (AAG) -Number AAG-PLI -Paragraph 132, 133, 135, 136, 137 -IssueDate 2006-09-01 -Chapter 4 -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. Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Statement of Position (SOP) -Number 94-5 -Paragraph 11, 12 -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 Amount of asbestos claims paid in the current period, net of reinsurance recoveries. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Audit and Accounting Guide (AAG) -Number AAG-PLI -Paragraph 132, 133, 135, 136, 137 -IssueDate 2006-09-01 -Chapter 4 -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. Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Statement of Position (SOP) -Number 94-5 -Paragraph 11, 12 -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 The increase (decrease) during the reporting period in asbestos and environmental claims reserve, net of reinsurance recoveries. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (b),(c) -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437 + Details
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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 The amount of the other adjustment made to the claims reserves during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Statement of Position (SOP) -Number 94-5 -Paragraph 11 -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. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -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 16, 17, 18, 19 -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 Provision for benefits, claims and claims settlement expenses incurred during the period net of the effects of contracts assumed and ceded. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.5) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 5 -Article 7 + Details
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- Definition The portion of reinsurance receivables representing the aggregate amount of estimated amounts due as of the balance sheet date from another insurer; also called reinsurance recoverables or unpaid losses recoverable, that are expected based upon statistical projections of incurred claims and claims settlement expenses which have not yet been reported to the insurer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 113 -Paragraph 14, 15 -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. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 6 -Article 7 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6485522&loc=d3e21412-158489 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6485522&loc=d3e21409-158489 + 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 survival ratio, which is the number of years that the carried A&E reserves will last assuming the current level of annual payments (based on a three-year average). + References No definition available. + Details
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- Definition Liability for Environmental Claims, Net, Incurred Loss + References No definition available. + Details
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- Definition Represents the net favorable loss reserve development due to the favorable development on property lines. + References No definition available. + Details
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- Definition Represents the net favorable loss reserve development due to the reduction from the loss reserve that was a result of the Chilean earthquake that occurred in 2010. + References No definition available. + Details
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- Definition Liabilty for Asbestos Claims, Net, Period Increase (Decrease) + References No definition available. + Details
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- Definition Liabilty for Environmental Claims, Net, Period Increase (Decrease) + References No definition available. + 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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- Definition Number of Asbestos Exposures + References No definition available. + Details
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- Definition Represents the amount of deferred gains recognized during the period, attributable to the collection of the recoverables that were due under the reinsurance contract. + References No definition available. + Details
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