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Loss and Loss Adjustment Expense Reserves (Details) (USD $) | 3 Months Ended | 6 Months Ended | 3 Months Ended | 6 Months Ended | 2 Months Ended | 5 Months Ended
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Jun. 30, 2011 | Jun. 30, 2010 | Jun. 30, 2011 | Jun. 30, 2010 | Jun. 30, 2011 OneBeacon Segment | Jun. 30, 2010 OneBeacon Segment | Jun. 30, 2011 OneBeacon Segment | Jun. 30, 2010 OneBeacon Segment | Jun. 03, 2011 White Mountains Re Segment | Jun. 03, 2010 White Mountains Re Segment | Jun. 03, 2011 White Mountains Re Segment | Jun. 03, 2010 White Mountains Re Segment
Loss and Loss Adjustment Expense Reserves |  |  |  |  |  |  |  |  |  |  |  |
Gross beginning balance | $ 5,759,800,000 | $ 6,660,700,000 | $ 5,736,800,000 | $ 6,379,200,000 |  |  |  |  |  |  |  |
Less beginning reinsurance recoverable on unpaid losses | (2,304,200,000) | (2,899,400,000) | (2,344,000,000) | (2,771,500,000) |  |  |  |  |  |  |  |
Net loss and LAE reserves | 3,455,600,000 | 3,761,300,000 | 3,392,800,000 | 3,607,700,000 |  |  |  |  |  |  |  |
Loss and LAE reserves acquired - Central National |  |  |  | 17,600,000 |  |  |  |  |  |  |  |
Loss and LAE reserves sold - One Beacon Personal Lines |  | (231,000,000) |  | (231,000,000) |  |  |  |  |  |  |  |
Loss and LAE incurred relating to Current year losses | 302,700,000 | 428,300,000 | 692,500,000 | 991,700,000 |  |  |  |  |  |  |  |
Loss and LAE incurred relating to Prior year losses | (10,300,000) | (26,200,000) | (27,500,000) | (37,700,000) | (10,100,000) | (18,300,000) | (15,200,000) | (24,300,000) | (200,000) | (7,900,000) | (12,300,000) | (13,400,000)
Total incurred losses and LAE | 292,400,000 | 402,100,000 | 665,000,000 | 954,000,000 |  |  |  |  |  |  |  |
Accretion of fair value adjustment to loss and LAE reserves | 2,100,000 | 2,100,000 | 4,200,000 | 4,200,000 |  |  |  |  |  |  |  |
Foreign currency translation adjustment to loss and LAE reserves | 10,100,000 | (22,400,000) | 31,200,000 | (33,200,000) |  |  |  |  |  |  |  |
Loss and LAE paid relating to Current year losses | (87,700,000) | (169,600,000) | (135,200,000) | (258,400,000) |  |  |  |  |  |  |  |
Loss and LAE paid relating to Prior year losses | (223,200,000) | (292,700,000) | (508,700,000) | (611,100,000) |  |  |  |  |  |  |  |
Total loss and LAE payments | (310,900,000) | (462,300,000) | (643,900,000) | (869,500,000) |  |  |  |  |  |  |  |
Net ending balance | 3,449,300,000 | 3,449,800,000 | 3,449,300,000 | 3,449,800,000 |  |  |  |  |  |  |  |
Plus ending reinsurance recoverable on unpaid losses | 2,193,100,000 | 2,748,100,000 | 2,193,100,000 | 2,748,100,000 |  |  |  |  |  |  |  |
Gross ending balance | 5,642,400,000 | 6,197,900,000 | 5,642,400,000 | 6,197,900,000 |  |  |  |  |  |  |  |
Net favorable loss reserve development | 10,300,000 | 26,200,000 | 27,500,000 | 37,700,000 | 10,100,000 | 18,300,000 | 15,200,000 | 24,300,000 | 200,000 | 7,900,000 | 12,300,000 | 13,400,000
Net favorable loss reserve development due to 2010 Chilean earthquake |  |  | 9,000,000 |  |  |  |  |  |  |  |  |
Net favorable loss reserve development due to 1999 aviation loss |  |  | 9,000,000 |  |  |  |  |  |  |  |  |
Net unfavorable loss reserve development due to Asbestos and Environmental reserves additions |  |  | 2,000,000 |  |  |  |  |  |  |  |  |
Net unfavorable loss reserve development from other business lines, mainly marine |  |  | 3,700,000 |  |  |  |  |  |  |  |  |
Favorable loss reserve development of commercial and personal auto reserves |  |  |  |  |  |  |  | 6,500,000 |  |  |  |
Pre-tax un-accreted adjustment |  |  | $ 16,800,000 |  |  |  |  |  |  |  |  |

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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 the current period 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 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. 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 provision in the period for claims incurred in the reporting period and related claims settlement costs. + References Reference 1: 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 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 (b) -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 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 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 (b) -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437 + 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 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 2: 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. 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. Reference 4: 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 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Financial Reporting Release (FRR) -Number 403 -Paragraph 04 -Subparagraph b Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 7 -Section 03 -Paragraph 13 -Subparagraph a Reference 7: 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 + 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 SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 5 -Article 7 Reference 2: 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 + Details
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- Definition Alternate concept name for the aggregate amount of policy reserves (provided for future obligations including unpaid claims and claims adjustment expenses) and policy benefits (liability for future policy benefits) as of the balance sheet date; grouped amount of all the liabilities associated with the company's insurance policies. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.13(a)(1)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 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 FASB -Name Statement of Financial Accounting Standard (FAS) -Number 5 -Paragraph 9 -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 4: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Statement of Position (SOP) -Number 94-5 -Paragraph 11 -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 SEC -Name Regulation S-X (SX) -Number 210 -Article 7 -Section 03 -Paragraph 13 -Subparagraph a + Details
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- Definition Represents the amount of accreted fair value adjustment to loss and LAE reserves related to purchase accounting. + Details
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- Definition Represents the amount of foreign currency translation adjustment to loss and LAE reserves. + Details
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- Definition Represents the net favorable loss reserve development due to aviation loss that occurred 1999. + Details
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- Definition Represents the favorable loss reserve development due to release of commercial and personal auto reserves associated with participation in involuntary auto pools. + Details
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- Definition Represents the amount of loss and loss adjustment expense reserves acquired. + Details
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- Definition Represents the net unfavorable loss reserve development due to additions to the Asbestos and Environmental reserves. + Details
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- Definition The estimated amount recoverable as of the balance sheet date from reinsurers for claims incurred but not yet paid by the ceding insurer and associated claims settlement expenses, including estimated amounts for claims incurred but not reported, net of any related valuation allowance. + Details
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