Municipal Bond Guarantee Insurance - Schedule of Net Written Premiums (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Earned premiums:
Gross earned premiums (1)	$ 220.1	$ 6.2	$ 455.4	$ 17.2
HG/BAM
Written premiums:
Direct	12.8	14.4	34.5	45.5
Assumed	0.0	0.0	4.5	0.1
Gross written premiums (1)	12.8	14.4	39.0	45.6
Earned premiums:
Direct	5.9	5.3	16.8	14.5
Assumed	0.8	0.9	2.8	2.7
Gross earned premiums (1)	$ 6.7	$ 6.2	$ 19.6	$ 17.2