Municipal Bond Guarantee Insurance - Schedule of Net Written Premiums (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Earned premiums:
Net earned premiums	$ 353.2	$ 220.1	$ 783.8	$ 455.4
HG/BAM
Written premiums:
Direct	18.4	12.8	44.9	34.5
Assumed	1.3	0.0	1.3	4.5
Net written premiums	19.7	12.8	46.2	39.0
Earned premiums:
Direct	6.5	5.9	22.0	16.8
Assumed	0.6	0.8	4.0	2.8
Net earned premiums	$ 7.1	$ 6.7	$ 26.0	$ 19.6