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Third Party Reinsurance (Details) (USD $) |  |  |  |  |  | 1 Months Ended | 3 Months Ended | 6 Months Ended | 122 Months Ended | 1 Months Ended | 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, 2011 | Dec. 31, 2010 | Jun. 30, 2011 OneBeacon | May 31, 2001 OneBeacon | Jun. 30, 2011 OneBeacon National Indemnity Company and General Reinsurance Corporation | May 31, 2001 OneBeacon National Indemnity Company NICO Cover | Jun. 30, 2011 OneBeacon National Indemnity Company NICO Cover | Jun. 30, 2011 OneBeacon National Indemnity Company NICO Cover | Jun. 30, 2011 OneBeacon National Indemnity Company NICO Cover | May 31, 2001 OneBeacon General Reinsurance Corporation GRC Cover | Jun. 30, 2011 OneBeacon General Reinsurance Corporation GRC Cover | Jun. 30, 2011 OneBeacon General Reinsurance Corporation GRC Cover | Jun. 30, 2011 OneBeacon Tower Insurance Company | Jun. 30, 2011 OneBeacon Tokio Marine and Nichido Fire | Jun. 30, 2011 OneBeacon Tokio Marine and Nichido Fire | Jun. 30, 2011 OneBeacon Hanover Insurance Company | Jun. 30, 2011 OneBeacon Munich Reinsurance America | Jun. 30, 2010 OneBeacon Northeast homeowners business | Jun. 30, 2010 OneBeacon Northeast homeowners business | Jun. 30, 2011 OneBeacon Property catastrophe reinsurance program | Jun. 30, 2011 White Mountains Re Segment | Jun. 30, 2011 White Mountains Re Segment General Reinsurance Corporation | Jun. 30, 2011 White Mountains Re Segment Lloyds of London | Jun. 30, 2011 White Mountains Re Segment Olympus | Jun. 30, 2011 White Mountains Re Segment Swiss Re Group | Jun. 30, 2011 White Mountains Re Segment Michigan Catastrophic Claims Association
Third party reinsurance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reinsurance recoverable on paid losses | $ 44,500,000 | $ 63,100,000 | $ 16,200,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 28,400,000 |  |  |  |  |
Reinsurance recoverable on unpaid losses | 2,193,100,000 | 2,344,000,000 | 1,995,800,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 367,200,000 |  |  |  |  |
Purchase accounting adjustments |  |  | 169,900,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of total paid and unpaid reinsurance recoverables (as a percent) |  |  |  |  | 74.00% |  |  |  |  |  |  |  | 3.00% | 3.00% | 3.00% | 6.00% | 2.00% |  |  |  |  | 11.00% | 11.00% | 11.00% | 10.00% | 4.00%
Reinsurance recoverables |  |  |  |  | 1,491,400,000 |  |  |  |  |  |  |  | 62,300,000 | 64,200,000 | 64,200,000 | 113,600,000 | 32,500,000 |  |  |  |  | 43,300,000 | 43,700,000 | 44,800,000 | 39,500,000 | 15,200,000
Third party recoverables which reinsurer would pay if they are unable to collect from third party reinsurers |  |  |  |  |  |  | 193,500,000 | 193,500,000 | 193,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reinsurance recoverable from various third party reinsurers, guaranteed by the third party reinsurer under reinsurance agreement |  |  |  |  |  |  |  |  |  |  |  |  |  | 37,100,000 | 37,100,000 |  |  |  |  |  |  |  |  |  |  |
The percentage ceded under quota share reinsurance agreement (in percent) |  |  |  |  |  |  |  |  |  |  |  |  |  | 100.00% | 100.00% |  |  | 30.00% |  |  |  |  |  |  |  |
Number of reinsurance contracts the seller caused OneBeacon to purchase immediately prior to the entity's acquisition of OneBeacon |  |  |  | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maximum cover under reinsurance contract |  |  |  |  |  | 2,500,000,000 | 2,500,000,000 | 2,500,000,000 |  | 570,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Adverse loss reserve development on losses occurring in years 2000 and prior |  |  |  |  |  |  |  |  |  | 400,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reserves ceded as of the date of the OneBeacon acquisition |  |  |  |  |  |  |  |  |  | 170,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Utilized coverage from NICO |  |  |  |  |  |  |  |  | 2,200,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reinsurance receivables paid by the reinsurer |  |  |  |  |  |  |  |  | 1,300,000,000 |  | 13,300,000 | 47,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of utilized coverage from NICO that relates to uncollectible Third Party Recoverables (as a percent) |  |  |  |  |  |  |  |  | 8.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Utilized coverage from NICO that relates to uncollectible Third Party Recoverables |  |  |  |  |  |  |  |  | 182,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maximum amount of loss on a single catastrophe retained, first layer |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 50,000,000 |  |  |  |  |  |
Amount of loss on a single catastrophe reinsured, second layer |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 175,000,000 |  |  |  |  |  |
First retention percentage of loss on a single catastrophe reinsured, second layer (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 26.00% |  |  |  |  |  |
Amount of loss on a single catastrophe reinsured, first retention slab for second layer, low end of range |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 50,000,000 |  |  |  |  |  |
Amount of loss on a single catastrophe reinsured, first retention slab for second layer, high end of range |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 100,000,000 |  |  |  |  |  |
Second retention percentage of loss on a single catastrophe reinsured, second layer (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 10.00% |  |  |  |  |  |
Amount of loss on a single catastrophe reinsured, second retention slab for second layer, low end of range |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 100,000,000 |  |  |  |  |  |
Amount of loss on a single catastrophe reinsured, second retention slab for second layer, high end of range |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 175,000,000 |  |  |  |  |  |
Minimum amount of loss on a single catastrophe retained, third layer |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 225,000,000 |  |  |  |  |  |
Premiums ceded under quota share agreement |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 14,000,000 | $ 25,600,000 |  |  |  |  |  |  |
Percentage collateralized (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2.00% | 3.00% | 100.00% | 4.00% |
Number of ratings in A.M. Best ratings | 15 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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- Definition Total consideration paid and payable to reinsurers during the period pursuant to terms of reinsurance arrangements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 17 -Article 12 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 113 -Paragraph 27 -Subparagraph a, 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 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b),(c)(2) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 + 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 Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6485522&loc=d3e21409-158489 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 7 -Section 03 -Paragraph 6 Reference 3: 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 4: 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 + Details
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- Definition Receivables currently due from reinsurers for ceded claims paid. + References Reference 1: 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 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.6) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 7 -Section 03 -Paragraph 6 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 -Subparagraph (a)(2) Reference 5: 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 6: 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 + Details
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- Definition Represents the amount of adverse loss reserve development on losses occurring in years 2000 and prior. + Details
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- Definition The number of ratings in the A.M. Best rating system. + Details
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- Definition Represents the amount of reserves ceded as of the date of the OneBeacon acquisition. + Details
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- Definition Represents the number of reinsurance contracts the seller caused OneBeacon to purchase from subsidiaries of Berkshire Hathaway Inc. immediately prior to the entity's acquisition of OneBeacon. + Details
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- Definition Amount of paid and unpaid reinsurance recoverables from the respective third party reinsurers. + Details
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- Definition Represents the percentage ceded under quota share reinsurance agreement. + Details
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- Definition The percentage of reinsurance recoverables collateralized through funds held, letters of credit or trust agreements. + Details
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- Definition The respective third party reinsurers' percentage share of the total paid and unpaid reinsurance recoverables. + Details
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- Definition First layer of reinsurance agreement that is retained by the entity. + Details
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- Definition The high end of the range of the retention for the second layer of the reinsurance agreement. + Details
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- Definition The low end of the range of the retention for the second layer of the reinsurance agreement. + Details
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- Definition Second layer of reinsurance agreement, in excess of the first layer, that is reinsured. + Details
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- Definition Represents the retention percentage for the second layer of the reinsurance agreement. + Details
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- Definition Represents the retention percentage for the second layer of the reinsurance agreement. + Details
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- Definition The high end of the range of the retention for the second layer of the reinsurance agreement. + Details
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- Definition The low end of the range of the retention for the second layer of the reinsurance agreement. + Details
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- Definition Third layer of reinsurance agreement, in excess of the second layer, that is retained by the entity. + Details
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- Details
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- Definition Represents amount of incurred losses paid by the reinsurer through the reporting date. + Details
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- Definition Represents the maximum cover amount under reinsurance contracts. + Details
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- Definition Purchase accounting adjustments related to reinsurance recoverables. + Details
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- Definition Reinsurance recoverables from various third party reinsurers that are guaranteed under the terms of a 100% quota share reinsurance agreement. + Details
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- Definition Amount of third party recoverables which reinsurer would pay if they are unable to collect from third party reinsurers. + Details
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- Definition Represents the utilized coverage from reinsurer. + Details
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- Definition Represents the utilized coverage from reinsurer that relates to uncollectible Third Party Recoverables. + Details
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- Definition Represents the percentage of utilized coverage from reinsurer that relates to uncollectible Third Party Recoverables. + Details
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