Municipal Bond Guarantee Insurance - Schedule of Net Written Premiums (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Earned premiums:
Net earned premiums	$ 227.8	$ 124.3	$ 430.6	$ 235.3
HG/BAM
Written premiums:
Direct	17.1	13.8	26.5	21.8
Assumed	0.0	0.0	0.0	4.5
Net written premiums	17.1	13.8	26.5	26.3
Earned premiums:
Direct	7.9	5.6	15.5	10.9
Assumed	2.6	0.9	3.4	2.0
Net earned premiums	$ 10.5	$ 6.5	$ 18.9	$ 12.9