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Summary of Significant Accounting Policies (Details 6) (USD $) In Millions, except Share data, unless otherwise specified |  |  |  |  | 1 Months Ended |  | 1 Months Ended |  |  |  |
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Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2011 OneBeacon Ltd. | Dec. 31, 2010 OneBeacon Ltd. | May 31, 2007 Sirius Group Non-cumulative perpetual preference shares director quarter | May 24, 2007 Sirius Group Non-cumulative perpetual preference shares | May 31, 2007 Sirius Group Non-cumulative perpetual preference shares Until June 30, 2017 | May 31, 2007 Sirius Group Non-cumulative perpetual preference shares After June 30,2017 | Dec. 31, 2011 White Mountains consolidated limited partnerships | Dec. 31, 2010 White Mountains consolidated limited partnerships | Dec. 31, 2011 A.W.G. Dewar Inc | Dec. 31, 2010 A.W.G. Dewar Inc
Noncontrolling Interest |  |  |  |  |  |  |  |  |  |  |  |
Noncontrolling ownership percentage in OneBeacon Ltd. |  |  | 24.50% | 24.00% |  |  |  |  |  |  |  |
Non-cumulative perpetual preference shares issued (in shares) |  |  |  |  | 250,000,000 |  |  |  |  |  |  |
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 |  |  |  |  |  |
Number of directors that can be elected to Sirius board |  |  |  |  | 2 |  |  |  |  |  |  |
Period for non-payment of dividend which give right to shareholders to elect directors on board (in quarters) |  |  |  |  | 6 |  |  |  |  |  |  |
Number of quarterly dividend payments which results in cessation of the right to elect directors |  |  |  |  | 4 |  |  |  |  |  |  |
Annual rate of dividend on preferred stock (as a percent) |  |  |  |  |  |  | 7.506% |  |  |  |  |
Reference rate (LIBOR) used to determine floating annual rate of the dividend on the Sirius Group preference shares |  |  |  |  |  |  |  | 3 month LIBOR |  |  |  |
Amount in excess of LIBOR reference rate used to determine floating dividend rate |  |  |  |  |  |  |  | 3.20% |  |  |  |
Floating annual dividend rate (if in excess of LIBOR plus 3.20%) |  |  |  |  |  |  |  | 7.506% |  |  |  |
Basis points on comparable treasury issue rate (as a percent) |  |  |  |  | 0.45% |  |  |  |  |  |  |
Noncontrolling equity interest | $ 57.1 | $ 62.8 |  |  |  |  |  |  | $ 54.8 | $ 59.7 | $ 2.3 | $ 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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