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Municipal Bond Guarantee Insurance - Schedule of Municipal Bond Guarantee Insured Obligations (Details) - Build America Mutual Assurance Company $ in Millions | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Mar. 31, 2023 | Dec. 31, 2023 USD ($) contract | Dec. 31, 2022 USD ($) contract
Guarantor Obligations [Line Items] |  |  |
Contracts outstanding | contract |  | 14,485 | 13,382
Remaining weighted average contract period (in years) | 10 years 9 months 18 days | 11 years 1 month 6 days |
Principal |  | $ 109,673.8 | $ 99,996.9
Interest |  | 56,012.2 | 48,880.6
Total debt service outstanding |  | 165,686.0 | 148,877.5
Gross unearned insurance premiums (in millions) |  | $ 325.8 | $ 298.3

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