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Statutory Capital and Surplus (Details) (USD $) | 12 Months Ended |  | 12 Months Ended |  |  | 12 Months Ended |  | 12 Months Ended |  | 12 Months Ended |  | 12 Months Ended |  | 12 Months Ended |  | 12 Months Ended |  | 12 Months Ended |  | 12 Months Ended |  | 12 Months Ended |  | 0 Months Ended | 12 Months Ended |  |  |  |  |  |  |  | 12 Months Ended |  |  |
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Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2013 Split Rock [Member] | Dec. 31, 2013 Sirius Group | Dec. 31, 2012 Sirius Group | Dec. 31, 2013 OneBeacon | Dec. 31, 2012 OneBeacon | Dec. 31, 2011 OneBeacon | Dec. 31, 2013 Other Operations | Dec. 31, 2012 Other Operations | Dec. 31, 2012 Regulated Operation OneBeacon | Dec. 31, 2013 Regulated Operation OneBeacon | Dec. 31, 2013 Unregulated Operation OneBeacon | Dec. 31, 2013 OneBeacon | Dec. 31, 2013 Sirius International | Dec. 31, 2012 Sirius International Sirius Group | Dec. 31, 2013 Sirius International Sirius Group | Dec. 31, 2012 Sirius International Regulated Operation Sirius Group | Dec. 31, 2013 Sirius International Regulated Operation Sirius Group | Dec. 31, 2013 HG Global | Dec. 31, 2013 HG Re | Dec. 31, 2013 BAM | Dec. 31, 2013 Subsidiaries | Dec. 31, 2013 One Beacon Insurance Company | Dec. 31, 2013 One Beacon Insurance Company Regulated Operation OneBeacon | Dec. 31, 2013 Split Rock [Member] | Dec. 31, 2012 White Mountains | Dec. 31, 2013 Atlantic Speciality Insurance Company (ASIC) [Member] | Dec. 31, 2012 Atlantic Speciality Insurance Company (ASIC) [Member] Regulated Operation OneBeacon | Dec. 31, 2013 Atlantic Speciality Insurance Company (ASIC) [Member] Regulated Operation OneBeacon | Dec. 31, 2013 Sirius America Sirius Group | Dec. 31, 2012 Sirius America Sirius Group | Dec. 31, 2011 Sirius America Sirius Group | Dec. 31, 2012 Sirius America Regulated Operation Sirius Group | Dec. 31, 2013 Central National Sirius Group | Dec. 31, 2012 Central National Sirius Group | Dec. 31, 2013 Woodridge Sirius Group | Dec. 31, 2012 Woodridge Sirius Group | Dec. 31, 2013 Oakwood Sirius Group | Dec. 31, 2012 Oakwood Sirius Group | Dec. 31, 2012 PICO Holdings Sirius Group | Dec. 31, 2013 Citation Sirius Group | Dec. 31, 2012 Citation Sirius Group | Dec. 31, 2013 Ashmere [Member] Sirius Group | Dec. 31, 2013 Empire [Member] Sirius Group | Dec. 31, 2013 White Shoals Reinsurance Company Sirius Group | Dec. 31, 2013 White Mountains Life Re | Dec. 31, 2013 White Mountains Advisors LLC | Dec. 31, 2013 Surplus Note BAM | Jan. 02, 2013 SWEDEN Sirius Group | Dec. 31, 2012 SWEDEN Sirius Group | Dec. 31, 2013 Preferred stocks HG Global | Dec. 31, 2013 Preferred stocks White Mountains HG Global | Dec. 31, 2013 SEK | Dec. 31, 2012 SEK | Dec. 31, 2011 SEK | Dec. 31, 2013 SEK Sirius International Sirius Group | Dec. 31, 2013 Common Stock [Member] OneBeacon | Dec. 31, 2013 Common Equities [Member] Other Operations | Dec. 31, 2013 Retained earnings | Dec. 31, 2012 Retained earnings | Dec. 31, 2011 Retained earnings | Dec. 31, 2010 Retained earnings | Dec. 31, 2013 Retained earnings Split Rock [Member] | Dec. 31, 2013 U.S and Bermuda [Member] OneBeacon | Dec. 31, 2012 U.S and Bermuda [Member] OneBeacon | Dec. 31, 2013 UNITED STATES OneBeacon
Statutory Accounting Practices [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Statutory capital and surplus |  |  | $ 96,400,000 |  |  | $ 866,200,000 |  |  |  |  |  | $ 900,000,000 |  |  |  |  | $ 2,500,000,000 |  |  |  | $ 436,900,000 | $ 469,000,000 |  |  |  |  |  |  |  | $ 700,000,000 | $ 548,400,000 | $ 528,300,000 |  |  | $ 9,300,000 | $ 11,200,000 | $ 8,400,000 | $ 8,600,000 | $ 20,700,000 | $ 24,500,000 |  | $ 11,900,000 | $ 13,100,000 | $ 12,700,000 | $ 10,700,000 | $ 14,600,000 | $ 85,900,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,000,000,000 | $ 900,000,000 | $ 900,000,000
Payments of Capital Distribution |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 190,000,000 |  |  |  | 190,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from Contributions from Affiliates |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 35,000,000 |  | 135,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Statutory net income (loss) |  |  |  |  |  | 100,100,000 | 82,000,000 | 119,600,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 55,900,000 | 26,200,000 | 101,400,000 |  | (500,000) | 1,700,000 | (100,000) | 100,000 | (900,000) | 3,900,000 |  | (300,000) | (4,200,000) | (400,000) | (10,400,000) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted equity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,700,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Surplus required |  |  |  |  |  | 164,700,000 |  |  |  |  |  |  |  |  |  |  | 132,400,000 |  |  |  |  | 66,000,000 |  |  |  |  |  |  |  |  | 113,900,000 |  |  |  | 1,500,000 |  | 0 |  | 2,200,000 |  | 6,300,000 | 100,000 |  | 4,600,000 | 1,400,000 | 6,200,000 | 700,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders' Equity Attributable to Parent | 3,905,500,000 | 3,731,800,000 |  | 1,459,600,000 | 1,559,700,000 | 830,600,000 | 763,100,000 |  | 1,025,800,000 | 802,800,000 |  |  |  |  |  |  |  |  |  |  |  |  | 2,800,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2,802,300,000 | 2,542,700,000 | 2,789,700,000 | 2,175,600,000 |  |  |  |
Amount available for dividend distributions without prior approval from regulatory authorities | 800,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 86,600,000 |  |  |  |  | 23,900,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 755,000,000 |  |  |  | 20,300,000 |  |  |
Statutory Accounting Practices, Statutory Capital, Percent |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 15.00% |  |  |
Dividends available, net of noncontrolling interest |  |  |  |  |  |  |  |  |  |  |  |  |  | 246,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |  |  | 600,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash Dividends Paid to Parent Company |  |  |  | 250,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash dividends declared to parent company by consolidated subsidiaries |  |  |  |  | 75,000,000 |  |  |  |  |  |  |  |  |  |  |  |  | 75,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Distributions made | 6,200,000 |  |  |  |  |  | 80,200,000 |  |  |  |  |  | 17,300,000 |  |  |  |  |  |  |  |  |  |  |  |  |  | 60,300,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net unrestricted cash, short-term investments and fixed maturity investments |  |  |  | 67,000,000 |  | 217,500,000 |  |  | 231,000,000 |  |  |  | 57,900,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 17,300,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Trading Securities | 5,347,300,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 89,800,000 | 490,200,000 |  |  |  |  |  |  |  |
Pre-tax income transferred to parent company |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 101,900,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unrestricted statutory surplus available for distribution |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 587,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash Dividends Paid to Parent Company by Consolidated Subsidiaries |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 326,400,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 75,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity held in insurance and reinsurance subsidiaries |  | 2,000,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Statutory accounting practices earned surplus |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 94,600,000 | 33,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Exchange rate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 6.43 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 6.4339 | 6.4973 | 6.8645 |  |  |  |  |  |  |  |  |  |  |
Effective income tax |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 22.00% | 26.30% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other long-term investments | 288,900,000 | 294,200,000 |  | 20,300,000 |  |  |  |  | 139,200,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Safety Reserve |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount of safety reserve |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,600,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 10,400,000,000 |  |  |  |  |  |  |  |  |  |
Deferred Tax Liabilities, Safety Reserve |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 357,200,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Statutory capital and surplus required, percent |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 125.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maximum contributions per bankruptcy agreement |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 200,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ratio limitation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 70.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Expense limitation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 90,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Preferred stock, value |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 613,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 97.30% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Preferred stock dividend rate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 6.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends Payable |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 55,200,000 | 53,700,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accrued interest on surplus notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 35,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Collateral Held in Supplement Trust |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 400,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 8.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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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 942 -SubTopic 235 -Section S99 -Paragraph 2 -Subparagraph (SX 210.9-06) -URI http://asc.fasb.org/extlink&oid=6882215&loc=d3e537971-122884 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 06 -Article 9 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph b -Article 12 Reference 4: 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=26873400&loc=d3e24072-122690 + Details
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