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Municipal Bond Guarantee Insurance - Schedule of change in the fair value of BAM surplus notes (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024	Dec. 31, 2024	Jul. 01, 2024
Changes In Nominal Value Of Surplus Notes [Roll Forward]
Interest income from BAM Surplus Notes	$ 7.5	$ 0.0
Operating Segments | Financial Guarantee (HG Global)
Changes In Nominal Value Of Surplus Notes [Roll Forward]
Beginning nominal value	495.7
Interest income from BAM Surplus Notes	7.5	$ 0.0
Payments of principal and interest	0.0
Ending nominal value	503.2
Changes In Fair Value Discount Of Surplus Notes [Roll Forward]
Beginning fair value discount	(114.0)
Change in fair value of BAM Surplus Notes	0.0
Ending fair value discount	(114.0)
BAM Surplus Notes, at fair value	$ 389.2		$ 381.7	$ 387.4

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- Definition

Fair value portion of an agreement for an unconditional promise by the maker to pay the holder a definite sum of money at a future date.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 825
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 10
-Publisher FASB
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The total amount of other operating income, the components of which are not separately disclosed on the income statement, from items that are associated with the entity's normal revenue producing operation.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 220
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