Investments - Summary of Amortized Cost and Estimated Fair Value of Available-for-Sale Investments, Excluding VIE Investments Held by Successor Ambac, by Contractual Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2025	Dec. 31, 2024
Debt Securities, Available-for-sale [Line Items]
Amortized Cost, Due in one year or less	$ 180,013
Amortized Cost, Due after one year through five years	32,409
Amortized Cost, Due after five years through ten years	47,801
Amortized Cost, Due after ten years	979
Amortized Cost, Total	261,202
Fixed income securities, amortized cost	269,848	$ 289,712
Estimated Fair Value, Due in one year or less	179,824
Estimated Fair Value, Due after one year through five years	31,588
Estimated Fair Value, Due after five years through ten years	47,619
Estimated Fair Value, Due after ten years	998
Estimated Fair Value due, Total	260,029
Debt Securities, Available-for-sale	122,295	157,020
Residential Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed income securities, amortized cost	1,604	2,475
Debt Securities, Available-for-sale	1,597	2,446
Collateralized Debt Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed income securities, amortized cost	1,963	3,131
Debt Securities, Available-for-sale	1,975	3,142
Asset-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed income securities, amortized cost	1,772	5,049
Debt Securities, Available-for-sale	$ 1,795	$ 5,061