Municipal Bond Guarantee - Schedule of Net Written Premiums (Details) - HG/BAM - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Direct written premiums	$ 7.9	$ 6.4
Assumed written premiums	0.0	0.0
Gross written premiums	7.9	6.4
Direct premiums earned	3.7	3.0
Assumed premiums earned	0.5	0.0
Gross earned premiums	$ 4.2	$ 3.0