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Municipal Bond Guarantee Insurance - Schedule of Net Written Premiums (Details) - HG Global/BAM - Segment - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Written premiums:
Direct	$ 58.6	$ 63.8	$ 51.0
Assumed	0.0	1.3	4.6
Gross written premiums	58.6	65.1	55.6
Earned premiums:
Direct	28.7	28.6	23.2
Assumed	2.5	4.7	3.7
Gross earned premiums	$ 31.2	$ 33.3	$ 26.9

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