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Third Party Reinsurance (Written and Earned Premiums) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010
Written premiums: |  |  |
Direct | $ 1,390.1 | $ 1,218.7 | $ 1,358.3
Assumed | 1,047.9 | 1,037.7 | 1,013.3
Gross written premiums | 2,438.0 | 2,256.4 | 2,371.6
Ceded | (311.1) | (278.0) | (338.1)
Net written premiums | 2,126.9 | 1,978.4 | 2,033.5
Earned premiums: |  |  |
Direct | 1,328.2 | 1,164.4 | 1,360.4
Assumed | 1,041.1 | 1,032.1 | 990.4
Gross earned premiums | 2,369.3 | 2,196.5 | 2,350.8
Ceded | (305.7) | (272.0) | (321.8)
Net earned premiums | 2,063.6 | 1,924.5 | 2,029.0
Losses and LAE: |  |  |
Direct | 784.4 | 631.8 | 736.5
Assumed | 553.5 | 637.0 | 729.5
Gross losses and LAE | 1,337.9 | 1,268.8 | 1,466.0
Ceded | (144.0) | (94.5) | (249.4)
Total incurred losses and LAE | 1,193.9 | 1,174.3 | 1,216.6
OneBeacon |  |  |
Written premiums: |  |  |
Direct | 1,204.0 | 1,079.2 | 1,236.7
Assumed | 55.2 | 49.1 | 55.8
Gross written premiums | 1,259.2 | 1,128.3 | 1,292.5
Ceded | (80.0) | (65.6) | (124.8)
Net written premiums | 1,179.2 | 1,062.7 | 1,167.7
Earned premiums: |  |  |
Direct | 1,158.3 | 1,035.9 | 1,242.5
Assumed | 52.8 | 42.3 | 60.7
Gross earned premiums | 1,211.1 | 1,078.2 | 1,303.2
Ceded | (79.1) | (66.0) | (122.1)
Net earned premiums | 1,132.0 | 1,012.2 | 1,181.1
Losses and LAE: |  |  |
Direct | 687.5 | 551.8 | 677.1
Assumed | 29.6 | 9.2 | 45.1
Gross losses and LAE | 717.1 | 561.0 | 722.2
Ceded | (67.1) | (12.7) | (36.6)
Total incurred losses and LAE | 650.0 | 548.3 | 685.6
Sirius Group |  |  |
Written premiums: |  |  |
Direct | 186.1 | 139.5 | 121.6
Assumed | 992.7 | 988.6 | 957.5
Gross written premiums | 1,178.8 | 1,128.1 | 1,079.1
Ceded | (231.1) | (212.4) | (213.3)
Net written premiums | 947.7 | 915.7 | 865.8
Earned premiums: |  |  |
Direct | 169.9 | 128.5 | 117.9
Assumed | 988.3 | 989.8 | 929.7
Gross earned premiums | 1,158.2 | 1,118.3 | 1,047.6
Ceded | (226.6) | (206.0) | (199.7)
Net earned premiums | 931.6 | 912.3 | 847.9
Losses and LAE: |  |  |
Direct | 96.9 | 80.0 | 59.4
Assumed | 523.9 | 627.8 | 684.4
Gross losses and LAE | 620.8 | 707.8 | 743.8
Ceded | (76.9) | (81.8) | (212.8)
Total incurred losses and LAE | $ 543.9 | $ 626.0 | $ 531.0

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- Definition Premiums for all insurance assumed from other insurers, that is premiums ceded to the entity by other insurers. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b)(2),(c)(2) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 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. + Details
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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 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 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. + 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 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 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 5 -Article 7 + 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 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 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 5 -Article 7 + 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 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 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 1 -Article 7 Reference 4: 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. + Details
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- Definition Premiums written for all insurance and reinsurance contracts, after adding premiums assumed from other insurers and subtracting any amounts ceded to another insurer. + 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) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 + Details
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