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Municipal Bond Guarantee Insurance - Narrative (Details) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 USD ($) yr | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2012 USD ($) | Dec. 31, 2018
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |
Percentage of discounted future cash flows | 10.00% |  |  |  |
Percentage of par value of policy reinsured | 15.00% |  |  |  |
Percentage of risk premium ceded | 60.00% |  |  |  |
Repayments of debt, principal | $ 365.0 | $ 404.6 | $ 76.1 |  |
Interest paid | 1.4 | $ 2.1 | $ 0.1 |  |
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 |  |  |  | 503.0 |
Assets Held-in-trust | $ 603.0 |  |  |  |
BAM |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |
Year of U.S. Treasury Rate | yr | 1 |  |  |  |
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% |  |  |
HG Global |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |
Percentage of par value of policy reinsured |  | 15.00% |  |  |
Collateral held in supplement trust | $ 715.1 | $ 465.4 |  |  |
Interest receivable | 126.0 |  |  |  |
BAM |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |
Surplus notes | 499.0 | 300.0 |  |  |
BAM | HG Global |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |
Surplus notes | $ 499.0 | $ 503.0 |  |  |
Surplus Note | BAM |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |
Interest rate (as a percent) | 3.78% | 3.54% |  |  |
Debt instrument, basis spread on variable rate | 30000.00% |  |  |  |
Surplus Note | BAM |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |
Repayments of debt, principal | $ 4.0 |  |  |  |
Interest paid | $ 1.0 |  |  |  |
Combination One | Surplus Note | BAM |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |
Number of years of change in interest rate | yr | 5 |  |  |  |
Combination Two | Surplus Note | BAM |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |
Number of years of change in interest rate | yr | 3 |  |  |  |
Fixed interest rate | Surplus Note | BAM |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |
Interest rate (as a percent) | 8.00% |  |  |  |
Series A BAM Surplus Note | HG Global |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |
Surplus notes | $ 203.0 |  |  |  |
Series B BAM Surplus Note | HG Global |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |
Surplus notes | $ 300.0 |  |  |  |
Forecast | Surplus Note | BAM |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |
Interest rate (as a percent) |  |  |  |  | 4.60%

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