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Significant Transactions Sale of Essentia (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011 | Mar. 31, 2013 Essentia OneBeacon | Dec. 31, 2012 Underwriting Agreement [Member] Essentia OneBeacon | Dec. 31, 2011 Underwriting Agreement [Member] Essentia OneBeacon
Unusual or Infrequent Item [Line Items] |  |  |  |  |  |
Gain (Loss) on Disposition of Stock in Subsidiary | $ 21.3 | $ 0 | $ 0 | $ 23.0 |  |
Gain (Loss) on Disposition of Stock in Subsidiary, After Tax |  |  |  | 15.0 |  |
Premiums Written, Net |  |  |  |  | $ 179.7 | $ 166.6
Premiums written, percentage of total |  |  |  |  | 8.00% | 8.00%

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- Definition Amount of gain (loss) on sale or disposal of equity in securities of subsidiaries. + 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 Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums written. + 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) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 + Details
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- Definition Gain (Loss) on Disposition of Stock in Subsidiary, After Tax + References No definition available. + Details
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- Definition Premiums Written, Percentage of Total + References No definition available. + Details
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