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Summary of Significant Accounting Policies (Details) (USD $) In Millions, except Share data, unless otherwise specified |  |  |  |  | 1 Months Ended |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Sep. 30, 2011 rating state | Dec. 31, 2010 | Sep. 30, 2011 OneBeacon Ltd. | Dec. 31, 2010 OneBeacon Ltd. | May 31, 2007 Sirius Group Non-cumulative perpetual preference shares | May 24, 2007 Sirius Group Non-cumulative perpetual preference shares | Sep. 30, 2011 Tuckerman Fund I, the Prospector Offshore Fund and the Prospector Turtle Fund | Dec. 31, 2010 Tuckerman Fund I, the Prospector Offshore Fund and the Prospector Turtle Fund | Sep. 30, 2011 A.W.G. Dewar Inc | Dec. 31, 2010 A.W.G. Dewar Inc | May 25, 2011 OneBeacon Restricted Stock Chairman and CEO OneBeacon Long-Term Incentive Plan
Basis of Presentation |  |  |  |  |  |  |  |  |  |  |
Ownership percentage of OneBeacon Ltd.'s outstanding common shares (as a percent) |  |  | 75.50% | 76.00% |  |  |  |  |  |  |
Number of states in which Esurance insurance directly writes insurance | 30 |  |  |  |  |  |  |  |  |  |
Noncontrolling ownership percentage in OneBeacon Ltd (as a percent) |  |  | 24.50% | 24.00% |  |  |  |  |  |  |
Issuance of restricted common shares (in shares) |  |  |  |  |  |  |  |  |  |  | 630,000
Non-cumulative perpetual preference shares issued (in shares) |  |  |  |  | 250,000 |  |  |  |  |  |
Non-cumulative perpetual preference shares per share liquidation preference (in dollars per share) |  |  |  |  |  | $ 1,000 |  |  |  |  |
Proceeds from non-cumulative perpetual preference shares |  |  |  |  | $ 245.7 |  |  |  |  |  |
Non-cumulative perpetual preference shares issuance cost and commission |  |  |  |  |  | 4.3 |  |  |  |  |
Noncontrolling equity interest | $ 54.6 | $ 62.8 |  |  |  |  | $ 51.8 | $ 59.7 | $ 2.2 | $ 3.1 |

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- Definition The per share liquidation preference (or restrictions) of nonredeemable preferred stock (or preferred stock redeemable solely at the option of the issuer) that has a preference in involuntary liquidation considerably in excess of the par or stated value of the shares. The liquidation preference is the difference between the preference in liquidation and the par or stated values of the share. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21475-112644 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21484-112644 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 5 -Section 02 -Paragraph 29 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(d)(1)) -URI http://asc.fasb.org/extlink&oid=6881521&loc=d3e23780-122690 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 4 -Section 08 -Paragraph d Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 129 -Paragraph 4, 6 -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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