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Municipal Bond Guarantee Insurance - Schedule of Net Written Premiums (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Earned premiums:
Gross earned premiums	$ 22.8	$ 16.3	$ 13.9
HG/BAM
Written premiums:
Direct	61.5	28.1	44.8
Assumed	0.2	10.6	8.1
Gross written premiums	61.7	38.7	52.9
Earned premiums:
Direct	19.4	13.6	13.6
Assumed	3.4	2.7	0.3
Gross earned premiums	$ 22.8	$ 16.3	$ 13.9

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