Financial Guarantee Insurance Contracts - Summary of Gross Premium Receivable Roll-Forward (Direct and Assumed Contracts) (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	Dec. 31, 2021
Financial Guarantee Insurance Contracts, Premium Receivable [Roll Forward]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	$ 1,054,661				$ 1,414,614				$ 1,054,661	$ 1,414,614	$ 1,305,250	$ 1,098,073
Premiums Written, Net	59,988	$ 115,154	$ 111,206	$ 96,422	90,736	$ 77,499	$ 53,229	$ 51,823
Allowance for Doubtful Accounts, Premiums and Other Receivables	142				69				142	69	0	$ 0
Premium Receivable, Credit Loss Expense (Reversal)									73	69	0
Premium Receivable, Allowance for Credit Loss, Writeoff									0	0	0
Premium Receivable, Allowance for Credit Loss, Recovery									0	0	$ 0
Past Due Premiums	$ 5,690				$ 510				$ 5,690	$ 510