Special Purpose Entities, Including Variable Interest Entities - Additional Information (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2024 USD ($) Entity	Sep. 30, 2023 USD ($)	Sep. 30, 2024 USD ($) Entity	Sep. 30, 2023 USD ($) consolidationVIE	Dec. 31, 2023 USD ($) Entity
Variable Interest Entities [Line Items]
Variable Interest Entity Change in Fair Value of Assets and Liabilities	$ 1,000,000	$ 1,000,000	$ 2,000,000	$ 4,000,000
Revenue from Contract with Customer, Excluding Assessed Tax	23,000,000	15,000,000	54,000,000	39,000,000
Realized Investment Gains (Losses)	(1,000,000)	1,000,000	3,000,000	(7,000,000)
Other Operating Income (Expense), Net	10,000,000	2,000,000	28,000,000	7,000,000
Fair value of special purpose entities	0		0
Variable Interest Entity, Measure of Activity, Income or Loss before Tax	3,000,000	$ 1,000,000	5,000,000	$ 0
Long-term debt	$ 518,000,000		518,000,000		$ 508,000,000
Number of Variable Interest Entities Consolidated During the Period	0	0
Number of Variable Interest Entities Deconsolidated During Period | consolidationVIE				1
Gain (loss) from consolidating VIEs	$ 0	$ 0	0	$ 0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	$ 54,000,000		$ 54,000,000		$ 40,000,000
Consolidated Entities [Member]
Variable Interest Entities [Line Items]
Number Of Consolidated Variable Interest Entities | Entity	6		6		6
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, Reclassification Adjustment from AOCI for Derecognition, before Tax	$ 0	0	$ 0	0
VIE Change in FV of Assets Liabilities, Net of Credit Risk Change	1,000,000	1,000,000	2,000,000	4,000,000
Gross Investment Income, Operating	3,000,000	1,000,000
Revenue from Contract with Customer, Excluding Assessed Tax	2,000,000		10,000,000
Interest Expense, Long-term Debt	(3,000,000)	(2,000,000)	(9,000,000)	(10,000,000)
Other Operating Income (Expense), Net	0	0	6,000,000	(1,000,000)
Long-term debt	$ 2,996,000,000		$ 2,996,000,000		$ 2,967,000,000
Ambac UK [Member] | Consolidated Entities [Member]
Variable Interest Entities [Line Items]
Number Of Consolidated Variable Interest Entities | Entity	4		4		4
Ambac UK [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Long-term debt	$ 2,841,000,000		$ 2,841,000,000		$ 2,808,000,000
Ambac Assurance [Member] | Consolidated Entities [Member]
Variable Interest Entities [Line Items]
Number Of Consolidated Variable Interest Entities | Entity	2		2		2
Ambac Assurance [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Long-term debt	$ 156,000,000		$ 156,000,000		$ 159,000,000
Fixed Income Investments [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Gross Investment Income, Operating			8,000,000	6,000,000
Realized Investment Gains (Losses)	$ 0	$ 0	$ 0	$ 0