Municipal Bond Guarantee Insurance - Schedule of Net Written Premiums (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Earned premiums:
Net earned premiums	$ 1,076.7	$ 664.2	$ 22.8
HG/BAM
Written premiums:
Direct	63.8	51.0	61.5
Assumed	1.3	4.6	0.2
Net written premiums	65.1	55.6	61.7
Earned premiums:
Direct	28.6	23.2	19.4
Assumed	4.7	3.7	3.4
Net earned premiums	33.3	26.9	22.8
HG Global-BAM
Earned premiums:
Net earned premiums	$ 33.3	$ 26.9	$ 22.8