Municipal Bond Guarantee - Schedule of Net Written Premiums (Details) - HG/BAM - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Direct written premiums	$ 10.2	$ 3.9	$ 30.6	$ 28.9
Assumed written premiums	10.6	0.0	10.6	0.0
Gross written premiums	20.8	3.9	41.2	28.9
Direct premiums earned	4.4	3.3	11.9	9.7
Assumed premiums earned	0.8	0.0	1.8	0.0
Gross earned premiums	$ 5.2	$ 3.3	$ 13.7	$ 9.7