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Municipal Bond Guarantee Insurance - Schedule of BAM's Insured Obligations (Details) $ in Millions | 12 Months Ended
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Dec. 31, 2017 USD ($) Contract | Dec. 31, 2016 USD ($) Contract
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |
Gross unearned insurance premiums | $ 136.8 | $ 82.9
BAM |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |
Contracts outstanding | Contract | 6,371 | 4,807
Remaining weighted average contract period (in years) | 10 years 11 months | 10 years 10 months
Principal | $ 42,090.6 | $ 33,057.3
Interest | 21,057.1 | 16,396.6
Total debt service outstanding | 63,147.7 | $ 49,453.9
Gross unearned insurance premiums | $ 136.8 |

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