Municipal Bond Guarantee Insurance - Schedule of Net Written Premiums (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Written premiums:
Direct		$ 16.5	$ 24.1	$ 37.3
Assumed		0.0	14.0	0.0
Gross written premiums		16.5	38.1	37.3
Earned premiums:
Direct		7.4	15.8	21.4
Assumed		0.5	8.5	1.9
Gross earned premiums		7.9	24.3	23.3
Financial Guarantee (HG Global)
Written premiums:
Direct	$ 0.0	0.0	0.0	0.0
Assumed	14.0	14.2	34.5	31.9
Gross written premiums	14.0	14.2	34.5	31.9
Earned premiums:
Direct	0.0	0.0	0.0	0.0
Assumed	7.5	6.6	21.5	19.4
Gross earned premiums	$ 7.5	6.6	21.5	19.4
BAM
Written premiums:
Direct		16.5	24.1	37.3
Assumed		0.0	0.0	0.0
Gross written premiums		16.5	24.1	37.3
Earned premiums:
Direct		7.4	15.8	21.4
Assumed		0.5	1.0	1.9
Gross earned premiums		7.9	16.8	23.3
Eliminations
Written premiums:
Direct		0.0	0.0	0.0
Assumed		(14.2)	(20.5)	(31.9)
Gross written premiums		(14.2)	(20.5)	(31.9)
Earned premiums:
Direct		0.0	0.0	0.0
Assumed		(6.6)	(14.0)	(19.4)
Gross earned premiums		$ (6.6)	$ (14.0)	$ (19.4)