Financial Guarantee Insurance Contracts - Effect of Reinsurance on Premiums Written and Earned (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2024	Sep. 30, 2024	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Premiums Written									$ (334,311)	$ (233,702)	$ (146,379)
Assumed Reinsurance Premiums Written									48,459	39,585	0
Ceded Reinsurance Premiums Written									(294,088)	(193,462)	(117,826)
Premiums written, net of reinsurance	$ 2,608	$ (32,754)	$ (32,289)	$ (26,247)	$ (36,750)	$ (24,768)	$ (9,120)	$ (9,187)	(88,682)	(79,825)	(28,553)
Direct Premiums Earned									298,121	185,470	70,577
Net premiums earned	18,931	$ 27,441	$ 27,054	$ 25,579	24,945	$ 12,187	$ 7,785	$ 6,995	99,005	51,911	13,869
Assumed Reinsurance Premiums Earned									57,081	18,354	0
Ceded Reinsurance Premiums Earned									256,197	151,913	$ 56,708
Reinsurance Recoverable, Allowance for Credit Loss	$ 100				$ 100				$ 100	$ 100