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Municipal Bond Guarantee - Schedule of Net Written Premiums (Details) - HG/BAM - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2019 | Mar. 31, 2018
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |
Direct written premiums | $ 7.9 | $ 6.4
Assumed written premiums | 0.0 | 0.0
Gross written premiums | 7.9 | 6.4
Direct premiums earned | 3.7 | 3.0
Assumed premiums earned | 0.5 | 0.0
Gross earned premiums | $ 4.2 | $ 3.0

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