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Third-Party Reinsurance (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2009
Written premiums: |  |  |
Direct | $ 1,217.1 | $ 1,623.1 | $ 2,079.7
Assumed | 1,036.8 | 1,015.2 | 947.8
Gross written premiums | 2,253.9 | 2,638.3 | 3,027.5
Ceded | (275.3) | (613.5) | (402.9)
Net written premiums | 1,978.6 | 2,024.8 | 2,624.6
Earned premiums: |  |  |
Direct | 1,267.0 | 1,766.6 | 2,128.9
Assumed | 1,032.9 | 993.9 | 994.3
Gross earned premiums | 2,299.9 | 2,760.5 | 3,123.2
Ceded | (372.1) | (508.7) | (405.6)
Net earned premiums | 1,927.8 | 2,251.8 | 2,717.6
Losses and LAE: |  |  |
Direct | 1,231.1 | 936.5 | 1,089.5
Assumed | 663.9 | 822.8 | 533.0
Gross losses and LAE | 1,895.0 | 1,759.3 | 1,622.5
Ceded | (688.1) | (370.1) | (198.4)
Total incurred losses and LAE | 1,206.9 | 1,389.2 | 1,424.1
OneBeacon |  |  |
Written premiums: |  |  |
Direct | 1,077.6 | 1,501.5 | 1,972.4
Assumed | 48.2 | 57.7 | 58.6
Gross written premiums | 1,125.8 | 1,559.2 | 2,031.0
Ceded | (62.9) | (400.2) | (213.2)
Net written premiums | 1,062.9 | 1,159.0 | 1,817.8
Earned premiums: |  |  |
Direct | 1,138.5 | 1,648.7 | 2,021.7
Assumed | 43.1 | 64.2 | 66.8
Gross earned premiums | 1,181.6 | 1,712.9 | 2,088.5
Ceded | (166.1) | (309.0) | (229.7)
Net earned premiums | 1,015.5 | 1,403.9 | 1,858.8
Losses and LAE: |  |  |
Direct | 1,151.1 | 877.1 | 1,020.7
Assumed | 36.1 | 138.4 | 40.8
Gross losses and LAE | 1,187.2 | 1,015.5 | 1,061.5
Ceded | (606.3) | (157.3) | (56.2)
Total incurred losses and LAE | 580.9 | 858.2 | 1,005.3
OneBeacon | OneBeacon Commercial Lines |  |  |
Written premiums: |  |  |
Ceded |  | (262.2) |
Earned premiums: |  |  |
Net earned premiums |  | 165.0 |
Losses and LAE: |  |  |
Total incurred losses and LAE |  | 86.5 |
Sirius |  |  |
Written premiums: |  |  |
Direct | 139.5 | 121.6 | 107.3
Assumed | 988.6 | 957.5 | 889.2
Gross written premiums | 1,128.1 | 1,079.1 | 996.5
Ceded | (212.4) | (213.3) | (189.7)
Net written premiums | 915.7 | 865.8 | 806.8
Earned premiums: |  |  |
Direct | 128.5 | 117.9 | 107.2
Assumed | 989.8 | 929.7 | 927.5
Gross earned premiums | 1,118.3 | 1,047.6 | 1,034.7
Ceded | (206.0) | (199.7) | (175.9)
Net earned premiums | 912.3 | 847.9 | 858.8
Losses and LAE: |  |  |
Direct | 80.0 | 59.4 | 68.8
Assumed | 627.8 | 684.4 | 492.2
Gross losses and LAE | 707.8 | 743.8 | 561.0
Ceded | (81.8) | (212.8) | (142.2)
Total incurred losses and LAE | $ 626.0 | $ 531.0 | $ 418.8

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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 Total consideration paid and payable by direct policyholders for insurance contracts sold by the Entity during the period. This amount does not include premium assumed from or ceded to other insurers. + 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)(1) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 + Details
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- Definition Total provision in the period for future policy benefits, claims incurred and costs incurred in the claims settlement process before the effects of reinsurance arrangements. + 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 Premiums earned on the income statement for all insurance and reinsurance contracts after subtracting any amounts ceded to another insurer and adding premiums assumed from other insurers. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04.13(c)) -URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 1 -Article 7 Reference 3: 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 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.1) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 + Details
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