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Municipal Bond Guarantee Insurance Schedule of Net Written Premiums (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Earned premiums:
Net earned premiums	$ 16.3	$ 13.9	$ 10.4
HG/BAM
Written premiums:
Direct	28.1	44.8	63.2
Assumed	10.6	8.1	0.0
Gross written premiums	38.7	52.9	63.2
Earned premiums:
Net earned premiums	13.6	13.6	9.4
Assumed	2.7	0.3	0.0
Net earned premiums	$ 16.3	$ 13.9	$ 9.4

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