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Significant Transactions (Formation of HG and BAM) (Details) (USD $) In Millions, unless otherwise specified | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2012 | Dec. 31, 2012
Significant Transactions |  |
Percentage of par value of policy reinsured |  | 15.00%
White Mountains |  |
Significant Transactions |  |
Funds to capitalize HG Global | $ 600.0 | $ 594.5
HG Global |  |
Significant Transactions |  |
Ownership interest (as a percent) |  | 97.30%
Surplus notes |  | 503.0
Statutory capital and surplus |  | 412.0
Percentage of par value of policy reinsured |  | 15.00%
Income (loss) from continuing operations |  | 14.2
Interest income (expense), surplus notes |  | 18.4
HG Global | Common shareholders’ equity |  |
Significant Transactions |  |
Ownership interest (as a percent) |  | 88.70%
HG Global | Redeemable preferred stocks |  |
Significant Transactions |  |
Ownership interest (as a percent) |  | 97.30%
Build America Mutual |  |
Significant Transactions |  |
Income (loss) from continuing operations |  | (36.3)
Interest income (expense), surplus notes |  | (18.4)
HG Global |  |
Significant Transactions |  |
Statutory capital and surplus |  | $ 412.0
Percentage of par value of policy reinsured |  | 15.00%

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