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Municipal Bond Guarantee Insurance (Details) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2016 USD ($) Contract | Sep. 30, 2016 USD ($) | Jun. 30, 2016 USD ($) | Mar. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) Contract | Sep. 30, 2015 USD ($) | Jun. 30, 2015 USD ($) | Mar. 31, 2015 USD ($) | Dec. 31, 2016 USD ($) Contract | Dec. 31, 2015 USD ($) Contract | Dec. 31, 2014 USD ($) | Dec. 31, 2012 USD ($)
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Percentage of par value of policy reinsured | 15.00% |  |  |  |  |  |  |  | 15.00% |  |  |
Pre-tax (loss) income from continuing operations | $ (66.8) | $ 20.0 | $ (2.1) | $ 8.5 | $ 246.3 | $ (86.9) | $ (5.1) | $ 3.0 | $ (40.4) | $ 157.3 | $ (29.4) |
Net investment income |  |  |  |  |  |  |  |  | 86.8 | $ 60.8 | 59.5 |
BAM |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Next Quarter | 1.9 |  |  |  |  |  |  |  | 1.9 |  |  |
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Next Twelve Months | 7.3 |  |  |  |  |  |  |  | 7.3 |  |  |
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Two | 7.0 |  |  |  |  |  |  |  | 7.0 |  |  |
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Three | 6.6 |  |  |  |  |  |  |  | 6.6 |  |  |
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Four | 6.3 |  |  |  |  |  |  |  | 6.3 |  |  |
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Five | 5.9 |  |  |  |  |  |  |  | 5.9 |  |  |
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Six Through Ten | 49.9 |  |  |  |  |  |  |  | 49.9 |  |  |
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Second Quarter | 1.8 |  |  |  |  |  |  |  | 1.8 |  |  |
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Third Quarter | 1.8 |  |  |  |  |  |  |  | 1.8 |  |  |
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Fourth Quarter | $ 1.8 |  |  |  |  |  |  |  | $ 1.8 |  |  |
Municipal bond contract | Contract | 4,807 |  |  |  | 3,103 |  |  |  | 4,807 | 3,103 |  |
Insured Financial Obligations with Credit Deterioration, Remaining Weighted Average Contract Period |  |  |  |  |  |  |  |  | 10 years 10 months | 12 years 10 months |  |
Municipal bond outstanding principal | $ 33,057.3 |  |  |  | $ 22,556.0 |  |  |  | $ 33,057.3 | $ 22,556.0 |  |
Municipal bond outstanding interest | 16,396.6 |  |  |  | 11,984.4 |  |  |  | 16,396.6 | 11,984.4 |  |
Financial Guarantee Insurance Contracts, Unearned Premium Revenue | 83.0 |  |  |  | 50.2 |  |  |  | 83.0 | 50.2 |  |
White Mountains |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Capital contributions |  |  |  |  |  |  |  |  |  |  |  | $ 594.5
HG Global |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Surplus notes | $ 300.0 |  |  |  |  |  |  |  | $ 300.0 |  |  | 503.0
BAM Management |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Capital contributions |  |  |  |  |  |  |  |  |  |  |  | $ 14.5
Preferred stocks | HG Global |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest (as a percent) | 96.90% |  |  |  |  |  |  |  | 96.90% |  |  |
Common Stock | HG Global |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest (as a percent) | 88.40% |  |  |  |  |  |  |  | 88.40% |  |  |
BAM |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Surplus notes | $ 503.0 |  |  |  |  |  |  |  | $ 503.0 |  |  |
Pre-tax (loss) income from continuing operations |  |  |  |  |  |  |  |  | (48.9) | (47.3) | (40.5) |
Net investment income |  |  |  |  |  |  |  |  | 6.8 | 4.2 | 5.7 |
HG Re |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Statutory capital and surplus | 467.2 |  |  |  | 452.9 |  |  |  | 467.2 | 452.9 |  |
Collateral Held in Supplement Trust | $ 465.4 |  |  |  | $ 441.0 |  |  |  | $ 465.4 | 441.0 |  |
HG Global |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Percentage of par value of policy reinsured | 15.00% |  |  |  |  |  |  |  | 15.00% |  |  |
Pre-tax (loss) income from continuing operations |  |  |  |  |  |  |  |  | $ 22.2 | 17.9 | 18.3 |
Net investment income |  |  |  |  |  |  |  |  | 2.2 | 1.9 | 1.4 |
Surplus Note |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net investment income |  |  |  |  |  |  |  |  | 0.0 | 0.0 | 0.0 |
Surplus Note | BAM |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net investment income |  |  |  |  |  |  |  |  | (17.8) | (15.8) | (15.7) |
Surplus Note | HG Global |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net investment income |  |  |  |  |  |  |  |  | 17.8 | $ 15.8 | $ 15.7 |
Interest Income [Member] | Surplus Note | HG Global |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Accrued Liabilities | $ 108.0 |  |  |  |  |  |  |  | $ 108.0 |  |  |

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