Fair Value Measurements - Summary of Carrying Amount and Fair Value of Ambac's Financial Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Financial assets:
Debt Securities, Available-for-sale	$ 122,295	$ 157,020
Other Investments	24,971	28,294
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	70,090	47,275	$ 19,223	$ 19,940
Derivative assets	0	0
Short-Term Debt	0	150,000
Financial liabilities:
Derivative liabilities	8	317
Fixed income securities, amortized cost	269,848	289,712
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	1,205	260
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	2,316	5,351
Short-term Investments [Member]
Financial assets:
Debt Securities, Available-for-sale	146,442	127,601
Financial liabilities:
Fixed income securities, amortized cost	146,434	127,588
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	8	13
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Asset-backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	1,795	5,061
Financial liabilities:
Fixed income securities, amortized cost	1,772	5,049
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	23	14
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	2
Collateralized Debt Obligations [Member]
Financial assets:
Debt Securities, Available-for-sale	1,975	3,142
Financial liabilities:
Fixed income securities, amortized cost	1,963	3,131
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	12	13
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	2
Residential Mortgage-Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	1,597	2,446
Financial liabilities:
Fixed income securities, amortized cost	1,604	2,475
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	7	29
US Government Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	35,424	40,995
Financial liabilities:
Fixed income securities, amortized cost	35,190	41,706
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	365	98
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	131	809
Corporate Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	66,573	89,192
Financial liabilities:
Fixed income securities, amortized cost	67,881	92,990
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	634	107
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	1,942	3,905
Municipal Bonds [Member]
Financial assets:
Debt Securities, Available-for-sale	11,590	14,083
Financial liabilities:
Fixed income securities, amortized cost	11,697	14,646
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	121	7
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	228	570
Commercial Mortgage Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	3,341	2,101
Financial liabilities:
Fixed income securities, amortized cost	3,307
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	42	8
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	8	34
Carrying Amount [Member]
Financial assets:
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	68,440	47,275
Loans	0	3,434
Total financial assets	362,148	363,624
Short-Term Debt	0	152,560
Financial liabilities:
Long-term Debt	118,901
Total financial liabilities	118,909	317
Cash and Cash Equivalents, Fair Value Disclosure		28,294
Carrying Amount [Member] | Other Debt Obligations [Member]
Financial assets:
Other Investments	24,971	127,601
Carrying Amount [Member] | Short-term Investments [Member]
Financial assets:
Short term investments	146,442
Carrying Amount [Member] | Foreign Exchange Forward
Financial liabilities:
Derivative liabilities	8
Total Fair Value [Member]
Financial assets:
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	68,440	47,275
Financial liabilities:
Cash and Cash Equivalents, Fair Value Disclosure		7,499
Fair Value Measured at Net Asset Value Per Share [Member]
Financial assets:
Other Investments	7,454	7,499
Level 1 [Member]
Financial assets:
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	68,440	47,275
Total financial assets	250,306	215,871
Short-Term Debt	0	0
Financial liabilities:
Long-term Debt	0
Total financial liabilities	0	0
Cash and Cash Equivalents, Fair Value Disclosure		0
Level 1 [Member] | Other Debt Obligations [Member]
Financial assets:
Other Investments	0	127,601
Level 1 [Member] | Short-term Investments [Member]
Financial assets:
Short term investments	146,442
Level 1 [Member] | Loans Receivable [Member]
Financial assets:
Loans	0	0
Level 1 [Member] | Foreign Exchange Forward
Financial liabilities:
Derivative liabilities	0
Level 2 [Member]
Financial assets:
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	0	0
Total financial assets	86,871	116,025
Short-Term Debt	0	0
Financial liabilities:
Long-term Debt	0
Total financial liabilities	8	317
Cash and Cash Equivalents, Fair Value Disclosure		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
Level 2 [Member] | Loans Receivable [Member]
Financial assets:
Loans	0	0
Level 2 [Member] | Foreign Exchange Forward
Financial liabilities:
Derivative liabilities	8
Level 3 [Member]
Financial assets:
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	0	0
Total financial assets	0	3,434
Short-Term Debt	0	152,560
Financial liabilities:
Long-term Debt	121,343
Total financial liabilities	121,343	152,560
Cash and Cash Equivalents, Fair Value Disclosure		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
Level 3 [Member] | Loans Receivable [Member]
Financial assets:
Loans	0	3,434
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	11,590	14,083
Financial liabilities:
Total financial liabilities		152,877
Corporate Debt Securities [Member] | Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	66,573	89,192
US Government Debt Securities [Member] | Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	35,424	40,995
Residential Mortgage-Backed Securities [Member] | Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	1,597	2,446
Collateralized Debt Obligations [Member] | Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	1,975	3,142
Asset-backed Securities [Member] | Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	1,795	5,061
Commercial Mortgage Backed Securities [Member] | Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	3,341	2,101
Fair Value, Recurring [Member]
Financial assets:
Total financial assets	344,631	342,829
Short-Term Debt	0	152,560
Financial liabilities:
Long-term Debt	121,343
Total financial liabilities	121,351	317
Fair Value, Recurring [Member] | Other Debt Obligations [Member]
Financial assets:
Other Investments	7,454	127,601
Fair Value, Recurring [Member] | Short-term Investments [Member]
Financial assets:
Short term investments	146,442
Fair Value, Recurring [Member] | Asset-backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	1,795	5,061
Fair Value, Recurring [Member] | Collateralized Debt Obligations [Member]
Financial assets:
Debt Securities, Available-for-sale	1,975	3,142
Fair Value, Recurring [Member] | Residential Mortgage-Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	1,597	2,446
Fair Value, Recurring [Member] | US Government Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	35,424	40,995
Fair Value, Recurring [Member] | Corporate Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	66,573	89,192
Fair Value, Recurring [Member] | Municipal Bonds [Member]
Financial assets:
Debt Securities, Available-for-sale	11,590	14,083
Financial liabilities:
Total financial liabilities		152,877
Fair Value, Recurring [Member] | Loans Receivable [Member]
Financial assets:
Loans	0	3,434
Fair Value, Recurring [Member] | Commercial Mortgage Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	3,341	2,101
Fair Value, Recurring [Member] | Foreign Exchange Forward
Financial liabilities:
Derivative liabilities	8
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	35,424	40,995
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
Financial liabilities:
Total financial liabilities		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	1,795	5,061
Fair Value, Recurring [Member] | Level 2 [Member] | Collateralized Debt Obligations [Member]
Financial assets:
Debt Securities, Available-for-sale	1,975	3,142
Fair Value, Recurring [Member] | Level 2 [Member] | Residential Mortgage-Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	1,597	2,446
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	66,573	89,192
Fair Value, Recurring [Member] | Level 2 [Member] | Municipal Bonds [Member]
Financial assets:
Debt Securities, Available-for-sale	11,590	14,083
Financial liabilities:
Total financial liabilities		317
Fair Value, Recurring [Member] | Level 2 [Member] | Commercial Mortgage Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	3,341	2,101
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	0
Fair Value, Recurring [Member] | Level 3 [Member] | Municipal Bonds [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] | Level 3 [Member] | Commercial Mortgage Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	$ 0	$ 0