Special Purpose Entities, Including Variable Interest Entities - Additional Information (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2023 USD ($) Entity consolidationVIE	Jun. 30, 2022 USD ($) consolidationVIE	Jun. 30, 2023 USD ($) Entity consolidationVIE	Jun. 30, 2022 USD ($)	Dec. 31, 2022 USD ($) Entity
Variable Interest Entities [Line Items]
Variable Interest Entity Change in Fair Value of Assets and Liabilities	$ 0	$ 1,000,000	$ 2,000,000	$ 1,000,000
Realized Investment Gains (Losses)	(3,000,000)	7,000,000	(8,000,000)	17,000,000
Other Operating Income (Expense), Net	2,000,000	0	5,000,000	3,000,000
Fair value of special purpose entities	0		0
Variable Interest Entity, Measure of Activity, Income or Loss before Tax	0	$ (6,000,000)	(1,000,000)	$ 15,000,000
Long-term debt	$ 501,000,000		$ 501,000,000		$ 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	0	0	1
Gain (loss) from consolidating VIEs	$ 0	$ 0	$ 0	$ 28,000,000
Number of Trusts Established Resulting From Puerto Rico Restructuring Transaction			2
Variable Interest Entity Deconsolidation Gain Loss	$ 0
Consolidated Entities [Member]
Variable Interest Entities [Line Items]
Number Of Consolidated Variable Interest Entities | Entity	8		8		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	$ 1,000,000	(1,000,000)	$ 1,000,000	(2,000,000)
VIE Change in FV of Assets Liabilities, Net of Credit Risk Change	0	0	3,000,000	(1,000,000)
Gross Investment Income, Operating	3,000,000	(1,000,000)
Interest Expense, Long-term Debt	(3,000,000)	(5,000,000)	(8,000,000)	(7,000,000)
Long-term debt	$ 2,956,000,000		$ 2,956,000,000		$ 3,107,000,000
Ambac UK [Member] | Consolidated Entities [Member]
Variable Interest Entities [Line Items]
Number Of Consolidated Variable Interest Entities | Entity	4		4		5
Ambac UK [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Long-term debt	$ 2,785,000,000		$ 2,785,000,000		$ 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	$ 171,000,000		$ 171,000,000		$ 319,000,000
Fixed Income Investments [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Gross Investment Income, Operating			5,000,000	(5,000,000)
Realized Investment Gains (Losses)	$ 0	$ 0	$ 0	$ 1,000,000