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Municipal Bond Guarantee - Schedule of Net Written Premiums (Details) - HG/BAM - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Direct written premiums	$ 9.7	$ 7.9
Assumed written premiums	0.0	0.0
Gross written premiums	9.7	7.9
Direct premiums earned	4.5	3.7
Assumed premiums earned	0.9	0.5
Gross earned premiums	$ 5.4	$ 4.2

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