Municipal Bond Guarantee Insurance - Schedule of Net Written Premiums (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Earned premiums:
Net earned premiums	$ 664.2	$ 22.8	$ 16.3
HG/BAM
Written premiums:
Direct	51.2	61.5	28.1
Assumed	4.6	0.2	10.6
Net written premiums	55.8	61.7	38.7
Earned premiums:
Direct	23.2	19.4	13.6
Assumed	3.7	3.4	2.7
Net earned premiums	26.9	22.8	16.3
HG Global-BAM
Earned premiums:
Net earned premiums	$ 26.9	$ 22.8	$ 16.3