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Municipal Bond Guarantee Insurance - Schedule of Municipal Bond Guarantee Insured Obligations (Details) - BAM $ in Millions | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Mar. 31, 2021 USD ($) Contract | Dec. 31, 2020 USD ($) Contract | Jan. 31, 2021 USD ($) | Sep. 30, 2019 USD ($)
Guarantor Obligations [Line Items] |  |  |  |
Contracts outstanding | Contract | 11,252 | 10,997 |  |
Remaining weighted average contract period outstanding (in years) | 10 years 7 months 6 days | 10 years 8 months 12 days |  |
Principal | $ 77,689.3 | $ 75,287.7 | $ 800.0 | $ 1,100.0
Interest | 36,710.5 | 36,448.8 |  |
Total debt service outstanding | 114,399.8 | 111,736.5 |  |
Total gross unearned insurance premiums | $ 243.7 | $ 237.5 |  |

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