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Significant Transactions (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended
--- | ---
Mar. 31, 2013 | Mar. 31, 2012
Significant Transactions |  |
Gain on sale of subsidiary - Essentia | $ 23.0 | $ 0
Excess of FV of aquired net assets over cost - American Fuji | 6.9 | 0
OneBeacon | Essentia |  |
Significant Transactions |  |
Gain on sale of subsidiary - Essentia | 23.0 |
Gain on sale of Essentia, after tax | 15.0 |
Premiums written | $ 35.3 |
Premiums written, percentage | 11.70% |

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- Definition Gain (loss) on entity's disposition of equity in securities of subsidiaries. Reflects the difference in the parent company's carrying amount of the equity interest in the subsidiary immediately before and after all stock transactions. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 13 -Subparagraph g -Article 9 + 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of Financial Accounting Standard (FAS) -Number 113 -Paragraph 27 -Subparagraph a, b -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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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