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Statutory Capital and Surplus (Details) | 12 Months Ended |  |  | 12 Months Ended |  | 12 Months Ended |  | 12 Months Ended |  | 12 Months Ended |  | 12 Months Ended |  |  |  | 1 Months Ended |
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Dec. 31, 2012 USD ($) | Dec. 31, 2011 USD ($) | Dec. 31, 2010 USD ($) | Dec. 31, 2012 Sirius Group USD ($) | Dec. 31, 2011 Sirius Group USD ($) | Dec. 31, 2012 OneBeacon USD ($) | Dec. 31, 2011 OneBeacon USD ($) | Dec. 31, 2010 OneBeacon USD ($) | Dec. 31, 2012 OneBeacon Ordinary Dividend USD ($) | Dec. 31, 2012 OneBeacon Dividends Received by White Mountains USD ($) | Dec. 31, 2012 Sirius Group USD ($) | Dec. 31, 2012 Sirius Group SEK | Dec. 31, 2012 Other Operations USD ($) | Dec. 31, 2011 Other Operations USD ($) | Dec. 31, 2012 Regulated Operation OneBeacon USD ($) | Dec. 31, 2012 Unregulated Operation OneBeacon USD ($) | Dec. 31, 2012 Sirius International Sirius Group USD ($) | Dec. 31, 2012 Sirius International Sirius Group SWEDEN USD ($) | Dec. 31, 2012 Sirius International Sirius Group USD ($) | Dec. 31, 2012 Sirius International Sirius Group SEK | Dec. 31, 2012 Sirius International Regulated Operation Sirius Group USD ($) | Dec. 31, 2012 HG Global USD ($) | Dec. 31, 2012 HG Re USD ($) | Dec. 31, 2012 BAM USD ($) | Dec. 31, 2012 Subsidiaries USD ($) | Dec. 31, 2012 One Beacon Insurance Company Regulated Operation OneBeacon USD ($) | Dec. 31, 2012 Atlantic Speciality Insurance Company (ASIC) [Member] Regulated Operation OneBeacon USD ($) | Dec. 31, 2012 Sirius America Sirius Group USD ($) | Dec. 31, 2011 Sirius America Sirius Group USD ($) | Dec. 31, 2010 Sirius America Sirius Group USD ($) | Dec. 31, 2012 Sirius America Regulated Operation Sirius Group USD ($) | Dec. 31, 2012 Central National Sirius Group USD ($) | Dec. 31, 2011 Central National Sirius Group USD ($) | Dec. 31, 2012 American General Sirius Group USD ($) | Dec. 31, 2012 American General Property Sirius Group USD ($) | Dec. 31, 2012 PICO Holdings Sirius Group USD ($) | Dec. 31, 2012 Citation Sirius Group USD ($) | Dec. 31, 2012 White Shoals Reinsurance Company Sirius Group USD ($) | Dec. 31, 2012 White Mountains Life Re USD ($) | Dec. 31, 2012 White Mountains Advisors LLC Other Operations USD ($) | Dec. 31, 2012 Surplus Note BAM | Dec. 31, 2012 SWEDEN Sirius Group | Dec. 31, 2011 SWEDEN Sirius Group | Dec. 31, 2012 Preferred stocks HG Global USD ($) | May 31, 2007 Preferred stocks Sirius Internation Group Ltd | Dec. 31, 2012 Preferred stocks White Mountains HG Global USD ($)
Disclosure of statutory capital and surplus |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Statutory capital and surplus |  |  |  |  |  | $ 900,000,000 | $ 1,000,000,000 |  |  |  |  |  |  |  | $ 900,000,000 |  |  | $ 2,500,000,000 |  |  |  | $ 412,000,000 | $ 412,000,000 | $ 483,700,000 |  |  | $ 700,000,000 | $ 528,300,000 | $ 533,700,000 |  |  | $ 11,200,000 | $ 13,400,000 | $ 8,600,000 | $ 24,500,000 | $ 6,300,000 | $ 13,100,000 | $ 15,500,000 | $ 33,700,000 |  |  |  |  |  |  |
Statutory net income (loss) |  |  |  |  |  | 82,000,000 | 119,600,000 | 257,200,000 |  |  |  |  |  |  | 329,900,000 |  |  |  |  |  |  |  |  |  |  |  |  | 26,200,000 | 101,400,000 | 70,200,000 |  | 1,700,000 | 100,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted equity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,600,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Surplus required |  |  |  |  |  | 207,500,000 |  |  |  |  |  |  |  |  |  |  |  | 109,200,000 |  |  |  |  |  | 65,000,000 |  |  |  | 127,800,000 |  |  |  | 1,700,000 |  | 100,000 | 2,500,000 | 200,000 | 900,000 | 7,000,000 | 400,000 |  |  |  |  |  |  |
Stockholders' Equity Attributable to Parent | 3,731,800,000 | 4,087,700,000 |  | 1,559,700,000 | 1,264,300,000 | 763,100,000 | 826,700,000 |  |  |  |  |  | 802,800,000 | 1,996,700,000 |  |  |  |  |  |  |  |  |  |  | 2,900,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount available for dividend distributions without prior approval from regulatory authorities | 1,200,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 329,900,000 |  | 15,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends available, net of noncontrolling interest | 268,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Statutory capital, balance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 528,300,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of prior year statutory surplus that may be paid as dividends without prior approval of regulatory authorities |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 10.00% |  |  |  |  |  |  |  |  |  |  |  | 10.00% | 10.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unassigned funds |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 700,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Represents dividends paid by OneBeacon's unregulated insurance operating subsidiaries to their immediate parent |  |  |  |  |  |  |  |  |  |  | 40,000,000 |  |  |  |  |  |  |  |  |  | 24,000,000 |  |  |  |  |  |  |  |  |  | 55,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash dividends declared to parent company by consolidated subsidiaries |  |  |  |  |  |  |  |  |  |  | 75,000,000 |  |  |  |  |  |  |  |  |  | 75,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash dividends paid and declared to parent company by consolidated subsidiaries |  |  |  |  |  |  |  |  |  |  | 115,000,000 |  |  |  |  |  |  |  |  |  | 99,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Distributions made |  |  |  |  |  |  |  |  | 80,000,000 | 60,000,000 |  |  |  |  | 173,100,000 | 4,900,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Distribution of Insurance Subsidiary to Parent |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 34,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net unrestricted cash, short-term investments and fixed maturity investments |  |  |  | 72,000,000 |  | 272,400,000 |  |  |  |  |  |  | 132,100,000 |  |  | 28,600,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 18,400,000 |  |  |  |  |  |
Common equity securities and other long-term investments |  |  |  |  |  | 33,300,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Pre-tax income transferred to parent company |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 82,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Pre-tax income intended to be transferred to parent company |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 110,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unrestricted statutory surplus available for distribution |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 852,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity held in insurance and reinsurance subsidiaries | 1,700,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount of Retained Earnings Available for Dividend without Approval | 800,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase in unrestricted statutory surplus due to reorganization |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 436,300,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash dividends inteded to paid to parent |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 50,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Statutory accounting practices earned surplus |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 56,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Exchange rate |  |  |  |  |  |  |  |  |  |  |  | 6.50 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other long-term investments | 294,200,000 | 301,300,000 |  | 18,000,000 |  |  |  |  |  |  |  |  | 77,600,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity securities |  |  |  |  |  |  |  |  |  |  |  |  | 540,400,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Safety Reserve |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount of safety reserve |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,500,000,000 | 9,600,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Safety reserve | 326,700,000 | 369,600,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 326,700,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Swedish tax rate (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 22.00% | 26.30% |  |  |
Safety reserve |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,500,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Statutory capital and surplus required, percent |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 125.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maximum contributions per bankruptcy agreement |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 200,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ratio limitation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 70.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Expense limitation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 110,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Preferred stock, value |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 613,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 97.30% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Preferred stock dividend rate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 6.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 7.506% |
Dividends Payable |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 16,800,000 |  | 16,300,000
Accrued interest on surplus notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 12,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Collateral Held in Supplement Trust |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 400,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 8.00% |  |  |  |  |
Accrued interest receivable on surplus notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 18,400,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash dividends declared and paid | $ 6,600,000 | $ 8,000,000 | $ 8,800,000 |  |  |  |  |  |  |  |  |  | $ 6,600,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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- Definition The amount of dividends that may be paid or declared in any calendar year without prior approval of regulatory agencies. + References Reference 1: 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.(e)) -URI http://asc.fasb.org/extlink&oid=6881521&loc=d3e23780-122690 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph e -Article 4 + Details
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- Definition This element represents disclosure of the aggregate cash dividends paid to the entity by consolidated subsidiaries. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-04.(b)) -URI http://asc.fasb.org/extlink&oid=6881521&loc=d3e24072-122690 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 235 -Section S99 -Paragraph 2 -Subparagraph (SX 210.9-06) -URI http://asc.fasb.org/extlink&oid=6882215&loc=d3e537971-122884 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 06 -Article 9 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph b -Article 12 + Details
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