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Municipal Bond Guarantee Insurance - Schedule of Net Written Premiums (Details) - HG Global/BAM - Segment - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Written premiums: |  |  |
Direct | $ 58.6 | $ 63.8 | $ 51.0
Assumed | 0.0 | 1.3 | 4.6
Gross written premiums | 58.6 | 65.1 | 55.6
Earned premiums: |  |  |
Direct | 28.7 | 28.6 | 23.2
Assumed | 2.5 | 4.7 | 3.7
Gross earned premiums | $ 31.2 | $ 33.3 | $ 26.9

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