Municipal Bond Guarantee Insurance Schedule of Net Written Premiums (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earned premiums:
Net earned premiums	$ 13.9	$ 10.4	$ 13.4
HG/BAM
Written premiums:
Direct	44.8	63.2	38.6
Assumed	8.1	0.0	0.0
Net written premiums	52.9	63.2	38.6
Earned premiums:
Net earned premiums	13.6	9.4	5.9
Assumed	0.3	0.0	0.0
Net earned premiums	$ 13.9	$ 9.4	$ 5.9