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Municipal Bond Guarantee (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 12 Months Ended | 1 Months Ended | 3 Months Ended | 12 Months Ended | 3 Months Ended |  |  |  |  |  | 3 Months Ended | 12 Months Ended | 3 Months Ended |
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Mar. 31, 2013 | Dec. 31, 2012 | Jul. 31, 2012 White Mountains | Sep. 30, 2012 White Mountains | Dec. 31, 2012 White Mountains | Mar. 31, 2013 HG Global | Dec. 31, 2012 HG Global | Mar. 31, 2013 HG Global Preferred Stock | Dec. 31, 2012 HG Global Preferred Stock | Mar. 31, 2013 HG Global Common Stock | Dec. 31, 2012 HG Global Common Stock | Mar. 31, 2013 Build America Mutual | Dec. 31, 2012 Non-controlling interest | Mar. 31, 2013 BAM | Dec. 31, 2012 HG Global
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Municipal bond contract | 105 | 3 |  |  |  |  |  |  |  |  |  |  |  |  |
Insured Financial Obligations with Credit Deterioration, Remaining Weighted Average Contract Period | 13 years 1 month 6 days | 10 years 4 months 24 days |  |  |  |  |  |  |  |  |  |  |  |  |
Municipal bond outstanding principal | $ 698.6 | $ 25.8 |  |  |  |  |  |  |  |  |  |  |  |  |
Municipal bond outstanding interest | 497.6 | 8.9 |  |  |  |  |  |  |  |  |  |  |  |  |
Funds to Capitalize HG Global |  |  |  | 600.0 | 594.5 |  |  |  |  |  |  |  | 14.5 |  |
Funds to Capitalized HG Global |  |  | 594.5 |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest (as a percent) |  |  |  |  |  |  |  | 97.30% | 97.30% | 88.70% | 88.70% |  |  |  |
Surplus Notes |  |  |  |  |  |  | 503.0 |  |  |  |  |  |  |  |
Statutory Accounting Practices, Statutory Capital and Surplus, Balance |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 417.7
Percentage of par value of policy reinsured | 15.00% |  |  |  |  |  |  |  |  | 15.00% |  |  |  |  |
Income (Loss) from Continuing Operations before Income Taxes, Foreign |  |  |  |  |  | 9.9 |  |  |  |  |  | 18.4 |  |  |
Interest income (expense), surplus notes |  |  |  |  |  | 10.1 |  |  |  |  |  |  |  | 10.1 |
Financial Guarantee Insurance Contracts, Unearned Premium Revenue | $ 2.0 | $ 0.1 |  |  |  |  |  |  |  |  |  |  |  |  |

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