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Loss and Loss Adjustment Expense Reserves (Loss and Loss Adjustment Expense) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 6 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | ---
Jun. 30, 2013 | Jun. 30, 2012 | Jun. 30, 2013 | Jun. 30, 2012 | Mar. 31, 2013 | Dec. 31, 2012 | Mar. 31, 2012 | Dec. 31, 2011
Loss and Loss Adjustment Expense Reserve |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gross beginning balance | $ 3,100.6 |  | $ 5,510.8 |  | $ 3,168.9 |  | $ 5,702.3 |  |  |  |  |  |  |  |  |
Less beginning reinsurance recoverable on unpaid losses | (410.8) |  | (2,465.4) |  | (429.1) |  | (2,507.3) |  |  |  |  |  |  |  |  |
Net loss and LAE reserves | 2,669.3 |  | 2,960.4 |  | 2,669.3 |  | 2,960.4 |  | 2,689.8 |  | 2,739.8 |  | 3,045.4 |  | 3,195.0 |
Loss and LAE reserves acquired - American Fuji | 0 |  | 0 |  | 21.3 |  |  |  |  |  |  |  |  |  |  |
Loss and LAE incurred relating to current year losses | 290.0 |  | 267.4 |  | 532.9 |  | 524.8 |  |  |  |  |  |  |  |  |
Net favorable and unfavorable loss reserve development | (15.4) |  | (3.5) |  | (14.0) |  | (11.1) |  |  |  |  |  |  |  |  |
Total incurred losses and LAE | 274.6 |  | 263.9 |  | 518.9 |  | 513.7 |  |  |  |  |  |  |  |  |
Accretion of fair value adjustment to net loss and LAE reserves | 0.1 |  | 1.2 |  | 1.3 |  | 8.3 |  |  |  |  |  |  |  |  |
Foreign currency translation adjustment to net loss and LAE reserves | (4.6) |  | (12.1) |  | (14.2) |  | (0.3) |  |  |  |  |  |  |  |  |
Current year losses | (83.3) |  | (78.4) |  | (119.9) |  | (122.9) |  |  |  |  |  |  |  |  |
Prior year losses | (207.3) |  | (222.5) |  | (477.9) |  | (545.4) |  |  |  |  |  |  |  |  |
Total loss and LAE payments | (290.6) |  | (300.9) |  | (597.8) |  | (668.3) |  |  |  |  |  |  |  |  |
Net loss and LAE reserves | 2,669.3 |  | 2,960.4 |  | 2,669.3 |  | 2,960.4 |  | 2,689.8 |  | 2,739.8 |  | 3,045.4 |  | 3,195.0 |
Reinsurance recoverable on unpaid losses | 388.6 |  | 2,369.4 |  | 388.6 |  | 2,369.4 |  |  |  |  |  |  |  |  |
Gross ending balance | 3,057.9 |  | 5,329.8 |  | 3,057.9 |  | 5,329.8 |  |  |  |  |  |  |  |  |
OneBeacon Runoff |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss and Loss Adjustment Expense Reserve |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net loss and LAE reserves | 0 | [1] | 296.0 | [1] | 0 | [1] | 296.0 | [1] | 0 | [1] | 0 | [1] | (333.1) | [1] | (384.0) | [1]
Net loss and LAE reserves | $ 0 | [1] | $ 296.0 | [1] | $ 0 | [1] | $ 296.0 | [1] | $ 0 | [1] | $ 0 | [1] | $ (333.1) | [1] | $ (384.0) | [1]
[1] | Loss and LAE reserve balances for OneBeacon's run-off business prior to September 30, 2012 were not classified as held for sale. Adjustment is to present loss and LAE reserve activities from continuing operations.

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- Definition Total amount of payments made during the reporting period to settle insured claims and pay costs incurred in the claims settlement process. + 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 SEC -Name Financial Reporting Release (FRR) -Number 403 -Paragraph 04 -Subparagraph b Reference 3: 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 + Details
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- Definition Amount of payments made in the reporting period to settle claims incurred in prior 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 (c) -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 c -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 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 Liability as of the balance sheet date for amounts representing estimated cost of settling unpaid claims under the terms of the underlying insurance policies, less estimated reinsurance recoveries on such claims. This includes an estimate for claims which have been incurred but not reported. Claim adjustment expenses represent the costs estimated to be incurred in the settlement of unpaid claims. + 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 SEC -Name Financial Reporting Release (FRR) -Number 403 -Paragraph 04 -Subparagraph b Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 13 -Subparagraph a -Article 7 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Statement of Position (SOP) -Number 01-5 -Paragraph 10 -Subparagraph a -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 5: 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. Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 60 -Paragraph 17, 18, 19, 20 -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 7: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 113 -Paragraph 21, 22, 23, 24, 25 -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 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 SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 6 -Article 7 Reference 2: 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 3: 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 Reference 4: 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. + Details
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