Investments - Summary of Gross Unrealized Losses and Fair Values of Ambac's Available-for-Sale Investments (Detail) $ in Millions	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	$ 4	$ 53
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	844	400
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	68	53
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	71	106
Debt Securities Accumulated Gross Unrealized Loss Before Tax Included Pledged Securities		106
Debt Securities, Available for sale, Unrealized Loss Position, Including Pledged Securities	1,032	1,269
Debt Securities , Available for sale, Continuous Unrealized Loss Position,	187
Available-for-sale Securities	$ 1,710	1,395
Average Percentage Loss Versus Amortized Cost	7.00%
Average Percentage of Unrealized Loss Versus Amortized Cost for Below Investment Grade Securities	5.00%
Municipal Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	$ 1	2
Available-for-sale Securities	72	43
US Government Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	4	4
Available-for-sale Securities	82	65
Corporate Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	44	63
Available-for-sale Securities	745	598
Debt Security, Government, Non-US [Member]
Debt Securities, Available-for-sale [Line Items]
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	6	9
Available-for-sale Securities	100	76
Residential Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	14	19
Available-for-sale Securities	250	238
Collateralized Debt Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	1	4
Available-for-sale Securities	139	137
Asset-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	1	5
Available-for-sale Securities	303	224
Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	4	78
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	8
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	4	86
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Available-for-sale Securities	$ 426	507
Ambac Insured Securities \| Parsed Fixed Income Securities
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-Sale, Unrealized Loss Position, Number of Positions	4,000
Commercial Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	$ 0	0
Available-for-sale Securities	19	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	184	791
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	4	53
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	844	392
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	68	53
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	1,028	1,183
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	71	106
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	7	21
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	16	7
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	1	1
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	23	28
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	1	2
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	27	40
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1	3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	37	19
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	2	1
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	63	58
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	4	4
Fixed Income Securities [Member] \| US Government Debt Securities [Member] \| Asset Pledged as Collateral [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities Available For Sale Continuous Unrealized Loss Position Less Than 12 Months Including Pledged Securities		869
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	75	280
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2	21
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	509	279
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	43	42
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	584	559
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	$ 44	63
Debt Securities, Available-for-Sale, Unrealized Loss Position, Number of Positions	662,000
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	$ 44
Fixed Income Securities [Member] \| Corporate Debt Securities [Member] \| External Credit Rating, Non Investment Grade [Member]
Debt Securities, Available-for-sale [Line Items]
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	1
Fixed Income Securities [Member] \| Debt Security, Government, Non-US [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	8	27
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	2
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	56	47
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	6	7
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	64	73
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	6	9
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	6	132
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	19
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	98	0
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	14	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	104	132
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	14	19
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	1	90
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	93	36
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	1	1
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	95	126
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	1	4
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	57	198
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1	4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	35	5
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	1	1
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	92	203
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	1	5
Fixed Income Securities [Member] \| Ambac Insured Securities
Debt Securities, Available-for-sale [Line Items]
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	$ 13
Debt Securities, Available-for-Sale, Unrealized Loss Position, Number of Positions	11,000
Dbt Securities Available for Sale Unrealized Loss Measured as a Percent of Amortized Cost	14.00%
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	$ 3	3
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	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	3	3
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Collateral Pledged [Member]
Debt Securities, Available-for-sale [Line Items]
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Available-for-sale Securities	27	64
Collateral Pledged [Member] \| Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Available-for-sale Securities	$ 27	$ 64

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Average Percentage of Unrealized Loss Versus Amortized Cost for Investment Grade Securities

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Debt Securities Accumulated Gross Unrealized Loss Before Tax Included Pledged Securities

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Debt Securities , Available for sale, Continuous Unrealized Loss Position, Less than 12 Months, Including Pledged Securities

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Debt Securities Available for Sale Unrealized Loss Measured as a Percent of Amortized Cost

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Debt Securities, Available for sale, Unrealized Loss Position, Including Pledged Securities

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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