Municipal Bond Guarantee Insurance - Narrative (Details)	1 Months Ended		3 Months Ended		9 Months Ended
	May 31, 2024 USD ($)	Dec. 31, 2023 USD ($)	Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)	Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)	Jul. 01, 2024 USD ($)
Guarantor Obligations [Line Items]
Member surplus contributions, insurance premium charges, percentage			55.00%		55.00%
Direct				$ 16,500,000	$ 38,100,000	$ 37,300,000
Gross earned premiums				7,900,000	24,300,000	23,300,000
Assets held-in-trust		$ 948,500,000	$ 976,400,000		976,400,000
Collateral trusts, investments and cash		588,600,000	644,700,000		644,700,000
Surplus notes		322,200,000	317,100,000		317,100,000
Interest receivable		37,700,000	14,600,000		14,600,000
Debt		564,600,000	544,900,000		544,900,000
Change in fair value of BAM Surplus Notes			$ 15,800,000		$ 15,800,000	0
BAM Surplus Notes | Surplus Notes
Guarantor Obligations [Line Items]
Basis spread on variable rate					3.00%
Stated margin			8.20%		8.20%		10.00%
Repayments of the BAM Surplus Notes subsequent to the deconsolidation of BAM	$ 8,000,000	27,400,000		0		0
Repayment of principal	5,100,000	17,800,000
Accrued interest payment held in trust	300,000	2,000,000
Accrued interest payment held outside of trust	$ 2,600,000	7,600,000
Debt		322,200,000	$ 317,100,000		$ 317,100,000
Interest receivable		174,500,000	$ 192,700,000		$ 192,700,000
BAM Surplus Notes | Surplus Notes | Discount Rate
Guarantor Obligations [Line Items]
Debt instrument, measurement input			0.0750		0.0750		0.0815
Financial Guarantee (HG Global)
Guarantor Obligations [Line Items]
Direct			$ 14,000,000.0	14,200,000	$ 34,500,000	31,900,000
Gross earned premiums			7,500,000	6,600,000	21,500,000	19,400,000
Financial Guarantee (HG Global) | Operating Segments
Guarantor Obligations [Line Items]
Debt		146,900,000	147,300,000		147,300,000
Notes receivable, fair value disclosure		0	411,100,000		411,100,000		$ 387,400,000
Change in fair value of BAM Surplus Notes			$ 15,800,000	$ 0	15,800,000	$ 0
Subsidiary - HG Global
Guarantor Obligations [Line Items]
Purchase of surplus notes					$ 503,000,000.0
Percentage of risk premium ceded			60.00%		60.00%
Renegotiation period					5 years
Direct			$ 14,000,000		$ 14,000,000
Gross earned premiums			7,500,000		$ 7,500,000
Percent over a target trust balance					102.00%
Subsidiary - HG Global | Regulation 114
Guarantor Obligations [Line Items]
Assets held-in-trust		341,600,000	363,200,000		$ 363,200,000
Supplement trust target			603,000,000		603,000,000
Subsidiary - HG Global | Supplemental Trust
Guarantor Obligations [Line Items]
Assets held-in-trust		$ 606,900,000	$ 613,200,000		$ 613,200,000
Subsidiary - HG Global | Maximum
Guarantor Obligations [Line Items]
Percentage of par value of policy reinsured			15.00%		15.00%
Variable Interest Entity, Primary Beneficiary - Build America Mutual Assurance Company | Fidus Re Ltd.
Guarantor Obligations [Line Items]
Reinsured amount					$ 125,000,000