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Municipal Bond Guarantee Insurance - Schedule of Net Written Premiums (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Earned premiums:
Gross earned premiums	$ 111.0	$ 5.4
HG/BAM
Written premiums:
Direct	8.0	9.7
Assumed	4.5	0.0
Gross written premiums	12.5	9.7
Earned premiums:
Direct	5.3	4.5
Assumed	1.1	0.9
Gross earned premiums	$ 6.4	$ 5.4

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