Municipal Bond Guarantee - Schedule of Net Written Premiums (Details) - HG/BAM - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Direct written premiums	$ 21.3	$ 12.5	$ 31.0	$ 20.4
Assumed written premiums	0.2	0.0	0.2	0.0
Gross written premiums	21.5	12.5	31.2	20.4
Direct premiums earned	4.7	3.8	9.2	7.5
Assumed premiums earned	0.9	0.5	1.8	1.0
Gross earned premiums	$ 5.6	$ 4.3	$ 11.0	$ 8.5