Financial Guarantee Insurance Contracts - Summary of Loss Reserve Roll-Forward, Net of Subrogation Recoverable and Reinsurance (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Loss And Loss Adjustment Expense Reserves [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	$ 72,626	$ 36,712	$ 9,072
Policyholder Benefits and Claims Incurred, Ceded	191,151	113,622	52,960
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Beginning balance of net loss and loss expense reserves	40,788	9,752	474
Current year:
Claim and loss expense payments, net of subrogation and reinsurance	(16,202)	(3,798)	206
Prior years:
Claim and loss expense (payments) recoveries, net of subrogation and reinsurance	(18,231)	(1,878)	0
Liability for Unpaid Claims and Claims Adjustment Expense, Net	78,981	40,788	9,752	$ 474
Liabillity for Claim and Claim Adjustment Expense, with Impact of VIE Consolidation	349,062	197,089	89,907	32,169
Current Year Claims and Claims Adjustment Expense	67,937	36,569	9,049
Prior Year Claims and Claims Adjustment Expense	4,689	143	23
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid	34,433	5,676	(206)
Ceded Loss And Loss Expenses Paid Not Yet Recovered		8,765	1,397
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	270,081	156,301	$ 80,155	$ 31,695
Liability for Claims and Claims Adjustment Expense	349,062	$ 197,089
Segment Reporting, Reconciling Item, Corporate Nonsegment | Other Insurance Contracts
Prior years:
Liability for Claims and Claims Adjustment Expense	$ 35,146