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Third Party Reinsurance (Sirius Group) (Details) (USD $) In Millions, unless otherwise specified |  |  |  |  |  | 3 Months Ended |
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Mar. 31, 2013 | Dec. 31, 2012 | Mar. 31, 2012 | Dec. 31, 2011 | Mar. 31, 2013 OneBeacon Rating | Mar. 31, 2013 Sirius Rating | Mar. 31, 2013 Berkshire Hathaway [Member] Sirius | Mar. 31, 2013 Swiss Re Group Sirius | Mar. 31, 2013 Olympus Sirius | Mar. 31, 2013 Lloyds of London Sirius | Mar. 31, 2013 Michigan Catastrophic Claims Association Sirius
Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reinsurance recoverable on paid losses | $ 15.6 | $ 17.9 |  |  | $ 2.4 | $ 13.2 |  |  |  |  |  |  |  |
Reinsurance recoverable on unpaid losses | 410.8 | 429.1 | 2,465.4 | 2,507.3 | 105.8 | 305.0 |  |  |  |  |  |  |  |
Reinsurance recoverables |  |  |  |  |  |  | 56.9 | 34.8 | 24.9 | [1] | 17.0 | [2] | 14.7 | [3]
Percentage of total paid and unpaid reinsurance recoverables |  |  |  |  |  |  | 18.00% | 11.00% | 8.00% | [1] | 5.00% | [2] | 5.00% | [3]
Percentage Collateralized |  |  |  |  |  |  | 0.00% | 0.00% | 100.00% | [1] | 9.00% | [2] | 0.00% | [3]
Number of ratings in A.M. Best ratings |  |  |  |  | 16 | 16 |  |  |  |  |  |  |  |
Seller Note From Acquisition |  |  |  |  |  | $ 25.0 |  |  |  |  |  |  |  |
[1] | Non-U.S. insurance entity. The balance is fully collateralized through funds held, letters of credit or trust agreements.
[2] | Represents the total of reinsurance recoverables due to Sirius Group from all Lloyds Syndicates.
[3] | Michigan Catastrophic Claims Association (“MCCA”) is a non-profit unincorporated association, established by the State of Michigan with the power to issue and collect assessments, to which every insurance company that sells automobile coverage in Michigan is required to be a member. A.M. Best does not rate MCCA. Sirius Group acquired its recoverable from MCCA in the acquisition of Stockbridge Insurance Company. As part of the acquisition, Sirius Group obtained $25.0 of reinsurance protection from the seller (currently rated A+ by A.M. Best) for unfavorable loss reserve development, including uncollectible reinsurance.

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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 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 FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 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. Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 6 -Article 7 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 The known and estimated amount recoverable as of the balance sheet date from reinsurers for claims paid or incurred by the ceding insurer and associated claims settlement expenses, including estimated amounts for claims incurred but not reported, and policy benefits, net of any related valuation allowance. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 113 -Paragraph 14, 16, 27, 113 -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 20 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6480758&loc=d3e6824-158387 Reference 4: 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.(a),6) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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- Definition Represents the percentage of a top reinsurer's recoverable amounts that are collateralized. + References No definition available. + Details
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- Definition The respective third party reinsurers' percentage share of the total paid and unpaid reinsurance recoverables. + References No definition available. + Details
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- Definition Seller note from acquisition of Stockbridge. + References No definition available. + Details
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