Special Purpose Entities, Including Variable Interest Entities - Additional Information (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2023 USD ($) Entity consolidationVIE	Sep. 30, 2022 USD ($) consolidationVIE	Sep. 30, 2023 USD ($) Entity consolidationVIE	Sep. 30, 2022 USD ($)	Dec. 31, 2022 USD ($) Entity
Variable Interest Entities [Line Items]
Variable Interest Entity Change in Fair Value of Assets and Liabilities	$ 1,000,000	$ (3,000,000)	$ 4,000,000	$ (2,000,000)
Realized Investment Gains (Losses)	1,000,000	14,000,000	(7,000,000)	31,000,000
Other Operating Income (Expense), Net	2,000,000	0	7,000,000	3,000,000
Fair value of special purpose entities	0		0
Variable Interest Entity, Measure of Activity, Income or Loss before Tax	1,000,000	$ (1,000,000)	0	$ 14,000,000
Long-term debt	$ 505,000,000		$ 505,000,000		$ 639,000,000
Number of Variable Interest Entities Consolidated During the Period	0	0	0	2
Number of Variable Interest Entities Deconsolidated During Period | consolidationVIE	1	0	2
Gain (loss) from consolidating VIEs	$ 0	$ 0	$ 0	$ 28,000,000
Number of Trusts Established Resulting From Puerto Rico Restructuring Transaction			2
Consolidated Entities [Member]
Variable Interest Entities [Line Items]
Number Of Consolidated Variable Interest Entities | Entity	7		7		9
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	1,000,000	$ 0	(1,000,000)
VIE Change in FV of Assets Liabilities, Net of Credit Risk Change	1,000,000	(2,000,000)	4,000,000	(3,000,000)
Gross Investment Income, Operating	1,000,000	1,000,000
Interest Expense, Long-term Debt	(2,000,000)	(1,000,000)	(10,000,000)	(8,000,000)
Other Operating Income (Expense), Net	0	0	1,000,000	(1,000,000)
Long-term debt	$ 2,699,000,000		$ 2,699,000,000		$ 3,107,000,000
Ambac UK [Member] | Consolidated Entities [Member]
Variable Interest Entities [Line Items]
Number Of Consolidated Variable Interest Entities | Entity	3		3		5
Ambac UK [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Long-term debt	$ 2,535,000,000		$ 2,535,000,000		$ 2,788,000,000
Ambac Assurance [Member] | Consolidated Entities [Member]
Variable Interest Entities [Line Items]
Number Of Consolidated Variable Interest Entities | Entity	4		4		4
Ambac Assurance [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Long-term debt	$ 164,000,000		$ 164,000,000		$ 319,000,000
Fixed Income Investments [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Gross Investment Income, Operating			6,000,000	(4,000,000)
Realized Investment Gains (Losses)	$ 0	$ 1,000,000	$ 0	$ 2,000,000