Variable Interest Entities - Summary of Carrying Amount of Assets, Liabilities and Maximum Exposure to Loss of Ambac's Variable Interests in Non-Consolidated Variable Interest Entities (Detail) - USD ($) $ in Thousands	Dec. 31, 2024	Dec. 31, 2023
Variable Interest Entities [Line Items]
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	$ 5,351	$ 6,594
Fixed income securities, amortized cost	289,712	341,685
Collateralized Debt Obligations [Member]
Variable Interest Entities [Line Items]
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	2	0
Fixed income securities, amortized cost	3,131	0
Residential Mortgage-Backed Securities [Member]
Variable Interest Entities [Line Items]
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	29	0
Fixed income securities, amortized cost	2,475	0
Municipal Bonds [Member]
Variable Interest Entities [Line Items]
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	570	696
Fixed income securities, amortized cost	$ 14,646	$ 9,394