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Third Party Reinsurance (Written and Earned Premiums) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011
Earned premiums: |  |  |  |  |  |
Net earned premiums | $ 1,987.3 |  | $ 2,063.6 |  | $ 1,924.5 |
Losses and LAE: |  |  |  |  |  |
Total incurred losses and LAE | 1,040.5 |  | 1,193.9 |  | 1,174.3 |
OneBeacon |  |  |  |  |  |
Written premiums: |  |  |  |  |  |
Direct | 1,103.1 |  | 1,204.0 |  | 1,079.2 |
Assumed | 59.8 |  | 55.2 |  | 49.1 |
Gross written premiums | 1,162.9 |  | 1,259.2 |  | 1,128.3 |
Ceded | (74.3) |  | (80.0) |  | (65.6) |
Net written premiums | 1,088.6 |  | 1,179.2 |  | 1,062.7 |
Earned premiums: |  |  |  |  |  |
Gross amount | 1,043.3 | [1] | 1,158.3 | [1] | 1,035.9 | [1]
Assumed from other companies | 148.5 | [1] | 52.8 | [1] | 42.3 | [1]
Gross earned premiums | 1,191.8 |  | 1,211.1 |  | 1,078.2 |
Ceded | (71.4) | [1] | (79.1) | [1] | (66.0) | [1]
Net earned premiums | 1,120.4 | [1] | 1,132.0 | [1] | 1,012.2 | [1]
Losses and LAE: |  |  |  |  |  |
Direct | 584.9 |  | 687.5 |  | 551.8 |
Policyholder Benefits and Claims Incurred, Assumed | 76.3 |  | 29.6 |  | 9.2 |
Gross losses and LAE | 661.2 |  | 717.1 |  | 561.0 |
Ceded | (39.1) |  | (67.1) |  | (12.7) |
Total incurred losses and LAE | 622.1 |  | 650.0 |  | 548.3 |
Sirius Group |  |  |  |  |  |
Written premiums: |  |  |  |  |  |
Direct | 177.3 |  | 186.1 |  | 139.5 |
Assumed | 943.1 |  | 992.7 |  | 988.6 |
Gross written premiums | 1,120.4 |  | 1,178.8 |  | 1,128.1 |
Ceded | (243.8) |  | (231.1) |  | (212.4) |
Net written premiums | 876.6 |  | 947.7 |  | 915.7 |
Earned premiums: |  |  |  |  |  |
Gross amount | 174.0 |  | 169.9 |  | 128.5 |
Assumed from other companies | 938.6 |  | 988.3 |  | 989.8 |
Gross earned premiums | 1,112.6 |  | 1,158.2 |  | 1,118.3 |
Ceded | (246.2) |  | (226.6) |  | (206.0) |
Net earned premiums | 866.4 |  | 931.6 |  | 912.3 |
Losses and LAE: |  |  |  |  |  |
Direct | 98.1 |  | 96.9 |  | 80.0 |
Policyholder Benefits and Claims Incurred, Assumed | 455.5 |  | 523.9 |  | 627.8 |
Gross losses and LAE | 553.6 |  | 620.8 |  | 707.8 |
Ceded | (135.2) |  | (76.9) |  | (81.8) |
Total incurred losses and LAE | 418.4 |  | 543.9 |  | 626.0 |
OneBeacon and Sirius Group |  |  |  |  |  |
Written premiums: |  |  |  |  |  |
Direct | 1,280.4 |  | 1,390.1 |  | 1,218.7 |
Assumed | 1,002.9 |  | 1,047.9 |  | 1,037.7 |
Gross written premiums | 2,283.3 |  | 2,438.0 |  | 2,256.4 |
Ceded | (318.1) |  | (311.1) |  | (278.0) |
Net written premiums | 1,965.2 |  | 2,126.9 |  | 1,978.4 |
Earned premiums: |  |  |  |  |  |
Gross amount | 1,217.3 |  | 1,328.2 |  | 1,164.4 |
Assumed from other companies | 1,087.1 |  | 1,041.1 |  | 1,032.1 |
Gross earned premiums | 2,304.4 |  | 2,369.3 |  | 2,196.5 |
Ceded | (317.6) |  | (305.7) |  | (272.0) |
Net earned premiums | 1,986.8 |  | 2,063.6 |  | 1,924.5 |
Losses and LAE: |  |  |  |  |  |
Direct | 683.0 |  | 784.4 |  | 631.8 |
Policyholder Benefits and Claims Incurred, Assumed | 531.8 |  | 553.5 |  | 637.0 |
Gross losses and LAE | 1,214.8 |  | 1,337.9 |  | 1,268.8 |
Ceded | (174.3) |  | (144.0) |  | (94.5) |
Total incurred losses and LAE | $ 1,040.5 |  | $ 1,193.9 |  | $ 1,174.3 |
[1] | The amounts shown excludes balances reclassified to held for sale in the consolidated balance sheets related to the Runoff Transaction as of December 31, 2013 and 2012 and to AutoOne as of December 31, 2011. See Note 21 - “Discontinued Operations”.

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