Fair Value Measurements - Summary of Carrying Amount and Fair Value of Ambac's Financial Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Financial assets:
Debt Securities, Available-for-sale	$ 157,020	$ 135,224
Other Investments	28,294	18,317
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	47,275	19,223	$ 19,940	$ 9,969
Derivative assets	0	656
Short-Term Debt	150,000
Financial liabilities:
Derivative liabilities	317	0
Fixed income securities, amortized cost	289,712	341,685
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	260	643
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	5,351	6,594
Equity Method Investments Excluded From Fair Value	177
Short-term Investments [Member]
Financial assets:
Debt Securities, Available-for-sale	127,601	200,510
Financial liabilities:
Fixed income securities, amortized cost	127,588	200,506
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	13	4
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Asset-backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	5,061	0
Financial liabilities:
Fixed income securities, amortized cost	5,049	0
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	14	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	2	0
Collateralized Debt Obligations [Member]
Financial assets:
Debt Securities, Available-for-sale	3,142	0
Financial liabilities:
Fixed income securities, amortized cost	3,131	0
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	13	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	2	0
Residential Mortgage-Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	2,446	0
Financial liabilities:
Fixed income securities, amortized cost	2,475	0
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	0	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	29	0
US Government Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	40,995	38,522
Financial liabilities:
Fixed income securities, amortized cost	41,706	39,091
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	98	364
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	809	933
Corporate Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	89,192	87,991
Financial liabilities:
Fixed income securities, amortized cost	92,990	92,694
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	107	262
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	3,905	4,965
Municipal Bonds [Member]
Financial assets:
Debt Securities, Available-for-sale	14,083	8,711
Financial liabilities:
Fixed income securities, amortized cost	14,646	9,394
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	7	13
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	570	696
Commercial Mortgage Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	2,101	0
Financial liabilities:
Fixed income securities, amortized cost	2,127
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	8	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	34	0
Warrant [Member]
Financial assets:
Derivative assets	0	656
Carrying Amount [Member]
Financial assets:
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	47,276	19,223
Loans	3,434	0
Total financial assets	363,625	373,930
Short-Term Debt	152,560	0
Financial liabilities:
Total financial liabilities	152,877	0
Carrying Amount [Member] | Other Debt Obligations [Member]
Financial assets:
Other Investments	28,294	18,317
Carrying Amount [Member] | Short-term Investments [Member]
Financial assets:
Short term investments	127,601	200,510
Carrying Amount [Member] | Warrant [Member]
Financial assets:
Derivative assets	0	656
Carrying Amount [Member] | Foreign Exchange Forward
Financial liabilities:
Derivative liabilities	317	0
Total Fair Value [Member]
Financial assets:
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	47,276	19,223
Fair Value Measured at Net Asset Value Per Share [Member]
Financial assets:
Other Investments	7,499	5,817
Level 1 [Member]
Financial assets:
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	47,276	19,223
Total financial assets	215,872	258,255
Short-Term Debt	0	0
Financial liabilities:
Total financial liabilities	0	0
Level 1 [Member] | Other Debt Obligations [Member]
Financial assets:
Other Investments	0	0
Level 1 [Member] | Short-term Investments [Member]
Financial assets:
Short term investments	127,601	200,510
Level 1 [Member] | Loans Receivable [Member]
Financial assets:
Loans	0	0
Level 1 [Member] | Foreign Exchange Forward
Financial liabilities:
Derivative liabilities	0	0
Level 2 [Member]
Financial assets:
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	0	0
Total financial assets	116,025	82,782
Short-Term Debt	0	0
Financial liabilities:
Total financial liabilities	317	0
Level 2 [Member] | Other Debt Obligations [Member]
Financial assets:
Other Investments	0	0
Level 2 [Member] | Short-term Investments [Member]
Financial assets:
Short term investments	0	0
Level 2 [Member] | Loans Receivable [Member]
Financial assets:
Loans	0	0
Level 2 [Member] | Foreign Exchange Forward
Financial liabilities:
Derivative liabilities	317	0
Level 3 [Member]
Financial assets:
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	0	0
Total financial assets	3,434	14,576
Short-Term Debt	152,560	0
Financial liabilities:
Total financial liabilities	152,560	0
Level 3 [Member] | Other Debt Obligations [Member]
Financial assets:
Other Investments	0	0
Level 3 [Member] | Short-term Investments [Member]
Financial assets:
Short term investments	0	0
Level 3 [Member] | Loans Receivable [Member]
Financial assets:
Loans	3,434	0
Level 3 [Member] | Warrant [Member]
Financial assets:
Derivative assets	0	656
Level 3 [Member] | Foreign Exchange Forward
Financial liabilities:
Derivative liabilities	0	0
Municipal Bonds [Member] | Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	14,083	8,711
Corporate Debt Securities [Member] | Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	89,192	87,991
US Government Debt Securities [Member] | Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	40,995	38,522
Residential Mortgage-Backed Securities [Member] | Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	2,446	0
Collateralized Debt Obligations [Member] | Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	3,142	0
Asset-backed Securities [Member] | Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	5,061	0
Commercial Mortgage Backed Securities [Member] | Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	2,101	0
Fair Value, Recurring [Member]
Financial assets:
Total financial assets	342,830	361,430
Short-Term Debt	152,560	0
Financial liabilities:
Total financial liabilities	152,877	0
Fair Value, Recurring [Member] | Other Debt Obligations [Member]
Financial assets:
Other Investments	7,499	5,817
Fair Value, Recurring [Member] | Short-term Investments [Member]
Financial assets:
Short term investments	127,601	200,510
Fair Value, Recurring [Member] | Asset-backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	5,061	0
Fair Value, Recurring [Member] | Collateralized Debt Obligations [Member]
Financial assets:
Debt Securities, Available-for-sale	3,142	0
Fair Value, Recurring [Member] | Residential Mortgage-Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	2,446	0
Fair Value, Recurring [Member] | US Government Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	40,995	38,522
Fair Value, Recurring [Member] | Corporate Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	89,192	87,991
Fair Value, Recurring [Member] | Municipal Bonds [Member]
Financial assets:
Debt Securities, Available-for-sale	14,083	8,711
Fair Value, Recurring [Member] | Loans Receivable [Member]
Financial assets:
Loans	3,434	0
Fair Value, Recurring [Member] | Commercial Mortgage Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	2,101	0
Fair Value, Recurring [Member] | Foreign Exchange Forward
Financial liabilities:
Derivative liabilities	317	0
Fair Value, Recurring [Member] | Level 1 [Member] | Asset-backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] | Level 1 [Member] | Collateralized Debt Obligations [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] | Level 1 [Member] | Residential Mortgage-Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] | Level 1 [Member] | US Government Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	40,995	38,522
Fair Value, Recurring [Member] | Level 1 [Member] | Corporate Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] | Level 1 [Member] | Municipal Bonds [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] | Level 1 [Member] | Commercial Mortgage Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] | Level 2 [Member] | Asset-backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	5,061	0
Fair Value, Recurring [Member] | Level 2 [Member] | Collateralized Debt Obligations [Member]
Financial assets:
Debt Securities, Available-for-sale	3,142	0
Fair Value, Recurring [Member] | Level 2 [Member] | Residential Mortgage-Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	2,446	0
Fair Value, Recurring [Member] | Level 2 [Member] | US Government Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] | Level 2 [Member] | Corporate Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	89,192	74,071
Fair Value, Recurring [Member] | Level 2 [Member] | Municipal Bonds [Member]
Financial assets:
Debt Securities, Available-for-sale	14,083	8,711
Fair Value, Recurring [Member] | Level 2 [Member] | Commercial Mortgage Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	2,101	0
Fair Value, Recurring [Member] | Level 3 [Member] | Asset-backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] | Level 3 [Member] | Collateralized Debt Obligations [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] | Level 3 [Member] | Residential Mortgage-Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] | Level 3 [Member] | US Government Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] | Level 3 [Member] | Corporate Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	0	13,920
Fair Value, Recurring [Member] | Level 3 [Member] | Municipal Bonds [Member]
Financial assets:
Debt Securities, Available-for-sale	0
Fair Value, Recurring [Member] | Level 3 [Member] | Commercial Mortgage Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	$ 0	$ 0