Investments - Summary of Gross Unrealized Losses and Fair Values of Ambac's Available-for-Sale Investments (Detail) $ in Thousands	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	$ 792	$ 103
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	68,012	87,006
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	4,559	6,491
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	5,351	6,594
Debt Securities, Available for sale, Unrealized Loss Position, Including Pledged Securities	119,024	107,338
Debt Securities , Available for sale, Continuous Unrealized Loss Position,	51,012	20,332
Available-for-sale Securities	$ 157,020	135,224
Average Percentage Loss Versus Amortized Cost	5.00%
Municipal Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	$ 570	696
Available-for-sale Securities	14,083	8,711
US Government Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	809	933
Available-for-sale Securities	40,995	38,522
Corporate Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	3,905	4,965
Available-for-sale Securities	89,192	87,991
Residential Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	29	0
Available-for-sale Securities	2,446	0
Collateralized Debt Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	2	0
Available-for-sale Securities	3,142	0
Asset-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	2	0
Available-for-sale Securities	5,061	0
Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	0	619
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0	0
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	0	619
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Available-for-sale Securities	127,601	200,510
Commercial Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	34	0
Available-for-sale Securities	2,101	0
Fixed Income Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	51,012	19,713
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	792	103
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	68,012	87,006
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	4,559	6,491
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	119,024	106,719
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	5,351	6,594
Fixed Income Securities [Member] | Municipal Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	6,042	732
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	112	17
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	6,582	7,015
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	458	679
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	12,624	7,747
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	570	696
Fixed Income Securities [Member] | US Government Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	15,919	6,847
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	344	37
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	14,818	20,981
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	465	896
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	30,737	27,828
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	809	933
Fixed Income Securities [Member] | Corporate Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	23,784	12,134
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	269	49
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	46,612	59,010
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	3,636	4,916
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	70,396	71,144
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	$ 3,905	4,965
Debt Securities, Available-for-Sale, Unrealized Loss Position, Number of Positions	145,000
Fixed Income Securities [Member] | Corporate Debt Securities [Member] | External Credit Rating, Investment Grade
Debt Securities, Available-for-sale [Line Items]
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	$ 3,905
Fixed Income Securities [Member] | Corporate Debt Securities [Member] | External Credit Rating, Noninvestment Grade [Member]
Debt Securities, Available-for-sale [Line Items]
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0
Fixed Income Securities [Member] | Residential Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	2,446	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	29	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0	0
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	2,446	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	29	0
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	655	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0	0
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	655	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	2	0
Fixed Income Securities [Member] | Asset-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	1,428	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0	0
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	1,428	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	2	0
Fixed Income Securities [Member] | Commercial Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	738	0
Fixed Income Securities [Member] | Collateralized Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	34	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0	0
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	738	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	$ 34	$ 0