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Statutory Financial Information (Detail) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2009
United States Traditional Insurance Subsidiaries [Member] |  |  |
Statutory Accounting Practices [Line Items] |  |  |
Statutory Net Income | $ 642.9 | $ 628.8 | $ 639.2
Statutory Combined Net Gain from Operations | 664.0 | 645.7 | 741.2
Tailwind Re and Northwind Re Subsidiaries [Member] |  |  |
Statutory Accounting Practices [Line Items] |  |  |
Statutory Net Income | 80.0 | 79.1 | 87.2
Statutory Combined Net Gain from Operations | $ 80.4 | $ 79.2 | $ 87.2

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- Definition Amount of net income for the period determined using accounting principles prescribed or permitted by insurance regulators. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 7 -Section 03 -Paragraph 24 -Subparagraph c Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.23(c)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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