Municipal Bond Guarantee Insurance - Schedule of Net Written Premiums (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Earned premiums:
Net earned premiums	$ 301.0	$ 227.8	$ 563.8	$ 430.6
HG/BAM
Written premiums:
Direct	11.6	17.1	20.8	26.5
Assumed	0.0	0.0	0.0	0.0
Net written premiums	11.6	17.1	20.8	26.5
Earned premiums:
Direct	7.0	7.9	14.0	15.5
Assumed	0.7	2.6	1.4	3.4
Net earned premiums	$ 7.7	$ 10.5	$ 15.4	$ 18.9