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Third Party Reinsurance (Sirius Group) (Details) (USD $) In Millions, unless otherwise specified | 9 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | ---
Sep. 30, 2012 | Mar. 31, 2012 | Dec. 31, 2011 | Sep. 30, 2011 | Mar. 31, 2011 | Dec. 31, 2010
Reinsurance [Line Items] |  |  |  |  |  |  |
Reinsurance recoverable on paid losses | $ 14.8 |  |  | $ 30.5 |  |  |
Reinsurance recoverable on unpaid losses | 356.8 |  | 2,369.4 | 2,507.3 | 2,581.2 | 2,193.0 | 2,344.0
Seller Note From Acquisition | 25.0 |  |  |  |  |  |
OneBeacon |  |  |  |  |  |  |
Reinsurance [Line Items] |  |  |  |  |  |  |
Reinsurance recoverable on paid losses | 2.0 |  |  |  |  |  |
Reinsurance recoverable on unpaid losses | 41.3 |  |  |  |  |  |
Reinsurance Recoverables | 1,449.2 |  |  |  |  |  |
Number of ratings in A.M. Best ratings | 15 |  |  |  |  |  |
Sirius |  |  |  |  |  |  |
Reinsurance [Line Items] |  |  |  |  |  |  |
Reinsurance recoverable on paid losses | 12.7 |  |  |  |  |  |
Reinsurance recoverable on unpaid losses | 315.5 |  |  |  |  |  |
General Reinsurance Corporation [Member] | Sirius |  |  |  |  |  |  |
Reinsurance [Line Items] |  |  |  |  |  |  |
Reinsurance Recoverables | 42.3 |  |  |  |  |  |
Percentage of total paid and unpaid reinsurance recoverables | 13.00% |  |  |  |  |  |
Percentage Collateralized | 1.00% |  |  |  |  |  |
Swiss Re Group [Member] | Sirius |  |  |  |  |  |  |
Reinsurance [Line Items] |  |  |  |  |  |  |
Reinsurance Recoverables | 33.7 |  |  |  |  |  |
Percentage of total paid and unpaid reinsurance recoverables | 10.00% |  |  |  |  |  |
Percentage Collateralized | 5.00% |  |  |  |  |  |
Olympus [Member] | Sirius |  |  |  |  |  |  |
Reinsurance [Line Items] |  |  |  |  |  |  |
Reinsurance Recoverables | 28.8 | [1] |  |  |  |  |
Percentage of total paid and unpaid reinsurance recoverables | 9.00% | [1] |  |  |  |  |
Percentage Collateralized | 100.00% | [1] |  |  |  |  |
Lloyds of London [Member] | Sirius |  |  |  |  |  |  |
Reinsurance [Line Items] |  |  |  |  |  |  |
Reinsurance Recoverables | 29.9 | [2] |  |  |  |  |
Percentage of total paid and unpaid reinsurance recoverables | 9.00% | [2] |  |  |  |  |
Percentage Collateralized | 4.00% | [2] |  |  |  |  |
Michigan Catastorphic Claims Association [Member] | Sirius |  |  |  |  |  |  |
Reinsurance [Line Items] |  |  |  |  |  |  |
Reinsurance Recoverables | $ 14.6 |  |  |  |  |  |
Percentage of total paid and unpaid reinsurance recoverables | 4.00% |  |  |  |  |  |
Percentage Collateralized | 0.00% |  |  |  |  |  |
[1] | (4) 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.
[2] | (3) Non-U.S. insurance entity. The balance is fully collateralized through funds held, letters of credit or trust agreements.

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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 The number of ratings in the A.M. Best rating system. + References No definition available. + 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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